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Volume 66, 1937
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The Embryo and Stelar Development of Histiopteris incisa

[Read before the Otago Branch, Royal Society of N.Z., November 12, 1935; received by the Editor, November, 15, 1935; issued separately, June, 1936.]

Contents.

Introduction.

The Gametophyte.

The Embryo.

Development of the Stele.

(a)

General.

(b)

Protostele.

(c)

Medullated Stele.

(d)

Lindsaya Stage.

(e)

Solenostele.

(f)

Leaf-trace and Root Departure.

The Mature Sporophyte.

(a)

Branching.

(b)

The Solenostele.

(c)

Cortex and Endodermis.

(d)

Leaf-trace and Pinna-trace.

Introduction.

The paper gives an account of the embryogeny and stelar development of Histiopteris incisa (Thunbg.) J. Smith together with a short account of certain other features in the life-history. This fern is widely distributed in southern tropical and temperate lands. Throughout the New Zealand botanical region it is abundant in light forest and on the forest edge.

The bulk of the sporophyte material was collected from damp roadside banks, and killed and fixed in formalin-acetic-70% alcohol. Young sporelings were obtained in the field growing on clay banks and fallen tree-fern stems, and a few were grown on prothalli in leaf-mould in the writer's cool greenhouse. The latter were less fibrous and gave better results in microtome sections. Sporeling stems for stages in the development of the stele were imbedded in paraffin and serial sections cut on the Cambridge rocker, usually at 10μ. These sections were stained in safranin and light green and studied from below upwards.

Prothalli were cultured from spores sown on sterilised flowerpots and tree-fern turves in Wardian cases. It was found necessary to repeat the sterilisation of both pots and turves after three weeks owing to the growth of fungi. After several months a few patches of fungal mycelium again appeared, but these were effectively killed by standing the turf to the depth of an inch in a solution of potassium permanganate (5 crystals to 500 c.c. water) for 15 minutes. The culture turves were watered from below with distilled water about once every three weeks, and the surface of the pots was kept moist by keeping a little water in the dish in which the upturned pot, filled with sphagnum moss, stood. When antheridia and archegonia

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appeared, the prothalli were thoroughly wetted by pipetting distilled water on to them once a week for four weeks. Young prothalli were collected at intervals and either drawn when fresh, or killed and fixed in 2% formalin and drawn later. To obtain sections of archegonia or embryos, prothalli were killed and fixed in strong chromacetic acid and embedded in paraffin. The sections, cut usually at 8μ on the Cambridge rocker, were stained with Delafield's Haematoxylin, and the stain differentiated with Scott's tap-water substitute.

Drawings were made by aid of an Abbé camera lucida or with a reflex drawing apparatus.

This paper forms part of a thesis submitted for the degree of Master of Arts of the University of New Zealand. The work was carried out at the Department of Botany, University of Otago, under the direction of the Rev. Dr J. E. Holloway, to whom the writer is deeply indebted for inspiration and encouragement.

The Gametophyte.

The ripe biconvex spores were sown on damp pots and tree-fern turves in February, and many germinated in a few days. Archegonia (Figs. 1–8) first appeared late in June. In several cases they were found on both surfaces of the prothallus, but much more abundantly on the convex under-side of the cushion. They continued to form until fertilisation of an egg took place, after which no more archegonia developed. In no case was more than one embryo found on a prothallus. Antheridia were not found until the end of July, possibly on account of the fact that the turves were kept too wet or in too humid an atmosphere. They developed amongst the rhizoids in the posterior region of the prothallus and at other places over the surface, even on the cushion near the archegonia. They continued forming while the archegonia and embryo were developing and also after the sporeling became established.

The Embryo.

Sections of over forty embryos were cut by the writer, and on these the following description is based. The fertilised egg surrounds itself by a membrane and follows the course of development characteristic in Leptosporangiate ferns. The surrounding prothallial cells divide to form a well-defined calyptra. The basal wall, or first wall of the fertilised egg, is vertical, that is, parallel to the axis of the archegonium and transverse to the axis of growth of the prothallus (Fig. 9). By it the egg is divided into anterior and posterior halves, the anterior half being that lying the nearer to the apex of the prothallus.

A stage showing the formation of the second or quadrant wall was not found, but in slightly older embryos it is quite evident that it is formed transversely to the axis of the archegonium and cutting the basal wall at right angles (Fig. 10).

A number of embryos were found at about the stage of development in Figs. 11–15. In these it is clearly seen that the quadrants develop in the following manner. The posterior lower quadrant divides irregularly to form the foot. The cells at the margin of

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the foot region come into contact with the calyptra, and later on it sometimes becomes difficult to determine the boundary between the foot and the prothallial tissue. In the posterior upper quadrant after a few divisions the root apical cell, a triangular pyramidal cell, is formed (Figs. 13, 15). It cuts off a segment in front which forms the first cell of the root-cap and then cuts off segments laterally on each of the three sides. It continues to segment and the root gradually elongates. In the anterior lower quadrant a few divisions take place, and then a triangular apical cell is formed, the stem initial, from which segments are cut off (Fig. 12). The stem initial is formed later than the cotyledon and root initials. The cotyledon initial is formed in the anterior upper quadrant as a broadly conical cell easily recognised from the much narrower stem initial (Figs. 11–15).

For a time the embryo is more or less spherical, but later the cotyledon initial grows rapidly and divides frequently (Fig. 16). The root also elongates almost as fast as the cotyledon. The foot increases in size by general cell division, but meanwhile the stem quadrant has grown extremely slowly. The young sporophyte lies vertically with its long axis parallel to the axis of the prothallus. The cotyledon soon bursts through the enlarged calyptra and extends upwards through the sinus of the prothallus to form the first frond. As it develops, the tip becomes involute as a result of the more rapid growth of the dorsal segments (Fig. 16) and later unrolls. It still continues to grow from a single apical cell which appears from the regular alternate arrangement of the segments to be cutting off segments from two sides. The root meanwhile extends downwards into the soil. The sporophyte is now independent, but the prothallus remains until two or three fronds have been formed.

Development of the Stele.

(a)

General.

During the development of the sporeling, the stem stele passes through the stages of (1) protostele, (2) medullated stele, and (3) Lindsaya stage before passing to the solenostelic structure. The stem stele of the sporeling is continuous with that of the first root, which is diarch in structure with usually two groups of three xylem tracheids and a few phloem elements in two groups. Sometimes the two groups of tracheids abut on each other at the centre of the stele so that there is a band of xylem; at other times the root stele is slightly larger and parenchyma cells are present between the two xylem groups. In the root the xylem and phloem are surrounded in succession by conjunctive parenchyma, pericycle, and endodermis. In the region of the foot a longitudinal section shows the stele passing smoothly and continuously from root to stem on the side opposite the foot, but on the foot side there is a prominent angle of vascular tissue projecting towards the foot region. This is indicated in Fig. 16 although the elements are not mature. A transverse section in this region shows the xylem elements on the foot side much wider than in either the sporeling stem, or first root, and usually cut obliquely. These larger xylem elements cut obliquely are a characteristic indication of the transition region.

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(b)

Protostele.

Above this transition region the stem stele is reached. The following two conditions were found in the sporeling stems examined. In some sporeling stems below the insertion of the first leaf-trace the stele is a protostele (Fig. 17) consisting of a solid central group of six xylem tracheids which may increase slightly higher up to seven or eight. The xylem group is cylindrical in outline, whereas in the root it forms a band. The phloem cells are at first few in number, surrounding the xylem in an interrupted ring, whereas in the root they are definitely in two groups. They can be recognised since they appear empty. Between the phloem and the xylem is a continuous, uniseriate ring of conjunctive parenchyma cells with dense contents. Outside the phloem is the pericycle, forming a complete ring of rather large cells, usually one cell layer in thickness, but in places occasionally two. The extra-stelar tissues consist of a well-marked uniseriate endodermis and a cortex three or four cells wide. In one case this protostelic stage persisted for twelve sections cut at a thickness of 10μ, but usually for a shorter distance. It is therefore of short duration compared with the protostele in other Filicales, and while it lasts there is practically no increase in the diameter of the stele or of the rhizome.

In certain other sporeling stems there was found a deviation from this structure. These stems from the very beginning are of slightly greater diameter and the xylem forms a continuous ring with parenchyma cells within it. The root in these cases shows parenchyma at the centre of the stele between the xylem groups, whereas in the cases previously described the xylem in the root forms a continuous band. The tissues surrounding the xylem in the stem are similar to those described for the previous example. The question arises as to whether this stele is to be regarded as a modified protostele or a medullated stele. The distinction between these is that a modified protostele has parenchyma cells indiscriminately mixed with the xylem, whereas a medullated stele has a definite parenchymatous medulla or pith; but in the case of Histiopteris a difficulty arises since the amount of xylem is so small that a modified protostele and medullated stele would be alike. However, since this stele is closely similar in appearance to the medullated stele following the protostele, and since in both the transition to the Lindsaya stage takes place in similar fashion, it seems preferable to regard it as a medullated stele. This means that in this case the protostelic stage is altogether omitted.

Bower (1911, p. 550) found in the Ophioglossaceae that the young plant may either have at first a solid xylem cylinder or have a small pith from the very beginning. The latter condition he relates to a more efficient nutrition of the young plant.

(c)

Medullated Stele.

In the case of protostelic stems, the transition to the medullated stage takes place below the point of departure of the first leaf-trace. First a parenchyma cell appears in the centre of the xylem core, and in the next few sections more of these cells appear until there are about six, the number depending on the increase in size of the

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stele (Figs. 18–19). The whole stele has during this time increased in diameter and there is a greater amount of phloem. Occasionally the xylem elements are not sufficient in number to form a complete-ring at this stage, and consequently the inner and outer parenchyma cells come into contact; at other times there is a complete ring of xylem. The medullated stele stage lasts a very short time, on an average only eighteen consecutive sections (sections 10μ thick). In the case where the protostele is absent, it of course is of longer-duration.

(d)

Lindsaya Stage.

When about six cells of inner conjunctive parenchyma are present within the xylem, internal phloem appears and the transition takes place to the Lindsaya stage. In Histiopteris first a phloem cell appears in the centre of the parenchyma cells (Fig. 20). It can be recognised by the fact that it is practically identical with the external phloem cells, appearing without contents, in contrast to the parenchyma cells with nuclei and abundant protoplasm. Typically at this stage the inner conjunctive parenchyma is uniseriate, though the cells may be two deep in places. There is a single ring of xylem, of outer conjunctive parenchyma, phloem, pericycle, and endodermis as we proceed from the centre outwards. About this stage, if the xylem is still a continuous ring, the ring breaks at one point to form the first leaf-gap. More internal phloem cells appear until there are about five. The first leaf-trace begins to depart and the stele temporarily increases in diameter to accommodate the leaf-trace, while the widening leaf-gap allows the internal and external parenchyma and the internal and external phloem to come into contact. A large number of stems of this age were examined, and in all cases it was found that the Lindsaya stage was reached before the first leaf-trace departed.

Above the departure of the first leaf-trace the stele becomes reduced in size, but always remains larger than it was at the medullated stage. The Lindsaya stage always persists for a considerable length of stem, but the actual length differs in different individual plants. It always lasts till the second leaf-trace has been passed, more frequently till the third leaf-trace has been passed, and occasionally beyond the fourth or even fifth leaf-trace. Proceeding upwards, there is generally a gradual increase in the size of the stele, with a temporary increase at the departure of the leaf-traces (Figs. 21, 22). The amount of xylem increases, the amount of external phloem increases, and there is an increase in the amount of phloem and parenchyma within the xylem. The diameter of the individual elements also gradually increases.

Occasionally after an increase for a time there is a gradual reduction in the diameter of the stele, and the amount of internal phloem is greatly reduced, although it was never found to disappear altogether. A somewhat similar behaviour was found by Bower (1911, p. 542) in weak plants of Botrychium ternata and B. lunaria, and to a more pronounced extent by Lang (1915, pp. 33–34) in some plants of Helminthostachys zeylanica where the rhizome, which had already attained the adult type of structure with tubular xylem,

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subsequently reverted to the juvenile protostelic condition by passing through a series of changes involving a diminution in size of the whole rhizome. Both Bower and Lang explain the phenomenon as probably due to growth under less favourable conditions of nutrition. A similar explanation would seem appropriate to the case in Histiopteris where diminution in the size of the stele and rhizome occurs; and on this view such reduction is to be regarded as abnormal. This conclusion is supported by the fact that reduction does not occur commonly. The specimens of Histiopteris in which reduction was found were obtained from dry clay banks, while the other specimens were collected from moist peaty banks and fallen tree-fern stems.

(e)

Solenostele.

The first appearance of the change from the Lindsaya condition occurs about midway between the departure of two successive leaf-traces, usually the third and fourth, when pericycle cells appear within the internal phloem (Fig. 23). They can be recognised by their large size and by the fact that they take the same characteristic purple stain with safranin and light green as the external pericycle cells do at this stage. Their number increases until about sixteen are present and then an endodermal cell appears in the centre of the pericycle cells. Provided the section is not cut from the region immediately behind the rhizome apex, the endodermal cell can readily be recognised by the fact that it stains red with safranin. The number of endodermal cells within the pericycle increases in the next few sections until five or six are present (Fig. 24). In the next section above, a thick-walled fibrous pith cell appears within the inner endodermal cells, and other similar cells soon appear (Fig. 25). Meanwhile all the other tissues, phloem, xylem, pericycle, endodermis, and cortex have been increasing in number and size of the individual elements, and consequently the stele and the whole rhizome have increased in diameter.

Once the pith appears within the xylem, a true leaf-gap is usually formed at the insertion of the next leaf-trace and the solenostelic stage is attained. The rhizome then consists from within outwards of (a) pith, (b) internal endodermis, (c) internal pericycle, (d) internal phloem, (e) internal conjunctive parenchyma, (f) xylem, (g) external conjunctive parenchyma, (h) external phloem, (i) external pericycle, (j) external endodermis, (k) cortex. Further growth and development consists in the expansion and increase of all the tissues.

(f)

Leaf-trace and Root Departure.

The creeping rhizome of the young sporophyte does not fork until at least the tenth frond has developed. All the roots except the first arise adventitiously and closely resemble the roots of the mature rhizome in origin and development.

The fronds are arranged alternately to the right and left of the mid-dorsal line in two dorso-lateral rows. At the departure of the first leaf-trace a distinct gap is formed in the xylem of the rhizome stele, a sector of the xylem ring accompanied by phloem

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  • and parenchyma passing out into the frond. The neighbouring pericycle and endodermal cells curve outwards with the outgoing xylem, with the result that there is complete continuity of the endodermis between rhizome and petiole. At the insertion of the leaf-trace, the internal phloem and parenchyma become continuous with their external counterparts. The next leaf-trace departs in similar fashion (Fig. 22) and in neither case is there any dipping in of pericycle or endodermis to form pockets.

    Bower (1918, p. 31) shows a figure of Histiopteris rhizome from Australian material at the leaf-gap immediately below the appearance of internal endodermis, with the endodermis and neighbouring cortical cells dipping inwards to form a shallow pocket. The writer examined seventeen rhizomes at this stage, and in no case could any trace be found of pocket formation.

    Above the appearance of internal endodermis the leaf-trace is similar with a single curved stele; but a distinct leaf-gap is formed in the rhizome stele and through it the outer tissues become continuous with the inner, endodermis with endodermis, pericycle with pericycle, phloem with phloem, and conjunctive parenchyma with conjunctive parenchyma. In one rhizome where continuity between external and internal endodermis was not established at the leaf-insertion immediately above the appearance of internal endodermis, a very slight endodermal pocket was formed, but this was the only indication of pocket formation. Later, when the rhizome branches, the leaf-trace insertion is similar to that in the mature plant.

    Compared with other solenostelic ferns such as Gleichenia pectinata and Loxsoma Cunninghamii (Bower, 1923, pp. 144, 145) the protostelic and medullated stages are passed through very rapidly and the Lindsaya stage is in proportion much elongated. Also the almost complete absence of pockets at the leaf-trace insertions is noteworthy.

The Mature Sporophyte.

(a)

Branching.

The large brown creeping rhizome of the sporophyte grows rapidly and branches repeatedly. On either side of the rhizome there runs longitudinally a lateral line of aerating tissue especially noticeable for about 4 cm. behind the apex. The branching appears to be a dichotomy, though frequently one of the branches is for a time smaller than the other. In the angle between the branches a frond arises, and in two cases it was found that a bud-shoot was developed from the lower part of the acroscopic margin of the petiole. The bud-shoot is of small diameter, and probably in normal cases it will not develop to any extent, but it is capable of developing strongly if the apex of the main rhizome is injured. The bud-shoot may have originated adventitiously, but appears similar to the bud-shoot of common occurrence in Hypolepis repens (Gwynne-Vaughan, 1903, Fig. 6), which is explained as being due to a second dichotomy.

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(b)

The Solenostele.

In transverse sections the mature rhizome has a concentric structure with a central pith surrounded by a ring-like stele and this in turn by the cortex. In older rhizomes the vascular strand may become corrugated on the ventral surface, but no medullary vascular developments were found in the pith.

The stele is of the type described by Gwynne-Vaughan (1903) as a solenostele. There is a xylem ring bounded both externally and internally by a complete ring of phloem, a pericycle, and an endodermis. The outer pericycle in small stems consists of a single layer of cells, but in larger stems it may be three-layered. From staining reactions it is concluded that mucilage is accumulated in the pericyclic cells, though at a later stage and more gradually than in the endodermal cells. The pericyclic cells usually contain starch grains, though these are much smaller than the starch in the cortex. The protophloem, best seen in young parts of the rhizome, appears as a single more or less continuous ring of narrow elements, while the phloem consists of a continuous ring one to two layers wide, the cells in transverse sections being readily recognised from their position and from the fact that they appear empty except for re-fringent granules on the walls. The conjunctive parenchyma occurs between the phloem and xylem in a more or less complete ring of thin-walled cells with abundant contents, including as a rule small starch grains; in older parts mucilage is present, and the contents then stain similarly to those of the pericycle. The xylem consists of a continuous ring of scalariform tracheids with a number of endarch protoxylem groups of spiral and annular tracheids. On the inner side of the xylem the tissues are similar in structure to those on the outer side.

(c)

Cortex and Endodermis.

The cortical cells are elongated, the length being three to fifteen times the diameter, and the end walls are oblique. The walls have simple pits and become brown as a result of impregnation with a substance which has been called phlobaphene. Several zones can be recognised in the cortex, including a zone of cells usually completely filled with large starch grains and having extremely thick and partly mucilaginous walls, and a zone of aerating tissue continued outwards laterally in the region of the mid-lateral lines on the outer surface, and consisting of thin-walled cells separated at the angles by large intercellular spaces. The walls bounding these intercellular spaces are densely fringed with short tubercles or rods which project into the air spaces. These rods appear similar to those described by Haberlandt (1928, p. 437) in Nephrodium stipellatum, which he considers have some function in connection with the ventilation system. The pith cells are similar to those of the cortex, being packed with starch and having cellulose walls impregnated with phlobaphene.

The stele is delimited from both the cortex and the pith by a well-marked uniseriate endodermis. Starch is never present in the endodermal cells, but the contents are evidently mucilaginous, as they have great avidity for stains. This fact is of practical value

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    Figs. 1–8.—Development of archegonium. × 140.
    Fig. 9.—Sagittal section of prothallus showing first wall in fertilised ovum. × 215.
    Fig. 10.—Ditto, showing quadrant walls and apical cell of cotyledon. × 215.
    Figs. 11–13.—Ditto, showing consecutive sections (8μ) of embryo. × 215. In fig. 12 ap. cell of root is not cut medianly, cal., calyptra; cot., cotyledon; f., foot. r., root; st., stem.

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    Figs. 14–15.—Two consecutive sections (8μ) of an embryo. × 215. In fig. 15 ap. cell of stem is cut obliquely.
    Fig. 16.—Older embryo in sagittal section of prothallus. × 162.5. Apex of cot. not quite median.
    Fig. 17.—T.S. of rhizome of very young sporophyte, showing protostele. × 275. c., cortex; c.p., conjunctive parenchyma; e., endodermis; per., pericycle; phl., phloem; x., xylem. × 275.
    Fig. 18.—Ditto, of same plant 6 sections (10μ) higher up, showing parenchyma (i. c.p.) within xylem. × 275.
    Fig. 19.—Ditto of same plant 6 sections higher up still. × 275.

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    Fig. 20.—T.S. of rhizome of same plant higher up still, showing beginning of internal phloem (i. phl.). × 275.
    Fig. 21.—T.S. of rhizome of somewhat older plant above first leaf-trace departure. × 275.
    Fig. 22.—Ditto of same plant, showing departure of second leaf-trace (1. tr.). × 275.
    Fig. 23.—Ditto of still older plant. i. per., internal pericycle. × 220.
    Fig. 24.—Ditto of same plant higher up. i.e., internal endodermal cells. × 220.

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    Fig. 25.—T.S. of rhizome of same plant as Figs. 23, 24, higher up still. p., thick-walled pith cells. × 220.
    Fig. 26.—T.S. of portion of solenostele of mature rhizome before pith walls have been thickened. i. pphl., inner protophloem; p., pith; pphl., protophloem; px., protoxylem. × 162.5.
    Fig. 27.—Sketch of rhizome at point of branching. br. 1, br. 2, branches of rhizome; pet., petiole. × ½.
    Fig. 28.—Stele of Fig. 27 after cortex dissected away, showing leaf gap in stem stele. × ½.

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  • as it enables the endodermis to be recognised at a glance. The radial walls in the primary condition are thickened by Caspary's bands, readily seen when stained with phloroglucin. On the inner tangential wall of all the endodermal cells and continuing for a short distance along the radial walls a layer of suberin is deposited as a lining on the inside of the cell wall and gives rise to the secondary stage of the endodermis. The suberin lamella develops close behind the rhizome apex. At 1&.5 cm. behind the apex small suberin globules appear closely aggregated on the inner tangential wall, and these gradually increase in size to form a continuous lamella 3 cm. behind the apex. In older parts, where the rhizome is approaching decay, the suberin lamella is broken in places.

    The endodermis in Histiopteris therefore seems similar to that found by Priestley and Radcliffe (1924) in Pteridium. They have carried out several experiments which they consider show that no sugar and very little water can pass across the continuous suberin lamella, but that water and sugar can pass across the lamella at both the granular and the broken stages. They further consider that starch is hydrolysed in the cortical cells especially in the spring, and that the sugar travels to the growing region by diffusion through the ground tissue. In this connection it might be mentioned that Conard (1908, p. 14) states for Dennstaedtia that “it may be that the inner cortex (of the root), whose walls thicken at such an early period, is for a time active in water conduction.”

    The writer was unable to carry out any convincing experiments on this very interesting subject, but a few points may be mentioned. Priestley's interpretation (1924) would perhaps account for the presence of mucilage in the endodermal cells and in the cortical walls, as, were water passing outwards from the stele only very slowly, the mucilage would hold it and conserve the supply. Moreover, the present writer was unable to obtain any indication of sugars in the endodermal cells. On the other hand, the starch grains in the cortex continue to increase in size after the suberin lamella is formed, and small starch grains are commonly found in the pericycle as if they were formed temporarily en route to the cortex. Besides sugar for starch formation, material for the cellulose thickening on the wall and for general metabolism must reach the cortical cells, and it does not seem likely that this extensive transference of material should take place only by way of the short immature region at the rhizome apices.

    Wherever the cortical cells are dead they contain no starch, and, when the older parts of the rhizome are gradually dying, it is found that the starch disappears from the cortical cells—commonly from the outer ones and a few groups of inner cortical cells first. Priestley (1924, p. 181) cut rhizomes of Pteridium in the autumn, waxed the ends, and left pieces in soil in a greenhouse till the spring, when he found that the starch completely disappeared if a young lateral frond was present.

    The writer tried a similar experiment with Histiopteris and in July dug up some rhizomes, and after waxing the cut ends with cacao butter replanted in soil in a greenhouse a portion with a young

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frond and branch developing. The adjacent pieces of rhizome, which were densely packed with starch, were killed and fixed to be kept for comparison later. A similar rhizome was cut and waxed in the forest without being removed from its position. When examined in October it was found that the branch had developed strongly and had branched in one case twice. Starch had been removed for a distance of about 27 mm. from the cut ends, and the cortical cells were here dead, but the rest of the cortex was still densely stocked with starch. However, in Histiopteris the chloroplasts are functioning in the young frond all the time it is uncurling, and the sugar manufactured may be sufficient for growth without tapping the starch reserves. The fact that in the developing petiole the suberin lamella is granular would without doubt enable water and solutes to pass from the vascular strand directly to the cortical cells of the petiole.

(d)

Leaf-trace and Pinna-trace.

The leaf-trace or stele of the basal part of the frond is divided into two or more parts, except in the case of very small fronds, and arises from the ventral portion of the rhizome stele. The portions of the divided leaf-trace gradually coalesce, and so the omega-shaped stele, characteristic of fern petioles, is formed some distance below the departure of the first pinna-trace. At the departure of the leaf-trace, the cortex and medulla of the rhizome come into contact through the leaf-gap, and at the margin of the gap the outer stelar tissues become continuous with their respective inner counterparts.

The pinna-trace, or stele of the primary pinna as it arises from the rachis, is of the marginal type with reinforcement from the abaxial curve of the petiolar stele. The vascular supply to the secondary pinnae is formed from the primary pinna-stele in the same way, but in the trace of the ultimate pinnules of the frond the supply is marginal with no reinforcement.

Summary.

The following points in the life history of Histiopteris incisa have been specially considered in this paper.

(a)

The embryo develops in the typical Leptosporangiate-fern manner. The first wall is vertical and the second is transverse, resulting in the early differentiation of the four main regions—foot, root, cotyledon, and stem. Apical cells are formed in all except the foot quadrant.

(b)

The stem stele passes through a protostelic, a medullated, and a Lindsaya stage before attaining the solenostelic structure. The protostelic stage is short or even absent, the medullated stage is also short, while the Lindsaya stage is in proportion much elongated.

(c)

The endodermis is similar to that of Pteridium with a suberin lamella developing on the inner tangential wall at an early stage. The starch reserves in the cortex are not lost to the plant, but whether or not passage of water or sugar can take place across the suberin lamella has not been determined.

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Bibliography.

Bower, F. O., 1911. On the Primary Xylem and the Origin of Medullation in the Ophioglossaceae, Annals of Botany, vol. xxv, pp. 1–553.

Bower, F. O., 1918. Studies in the Phylogeny of the Filicales VII, The Pteroideae, Annals of Botany, vol. xxxii, pp. 1–68, figs. 1–43.

Bower, F. O., 1923. The Ferns, Vol. I, Cambridge.

Bower, F. O., 1928. The Ferns, Vol. III, Cambridge.

Conard, H. S., 1908.—The Structure and Life History of the Hay-scented Fern, pp. 1–56, pl. 1–25, Washington.

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