General Form and Structure of the Prothallus.
The prothallus-body is cylindrical in form, being radially constructed. It is brown in colour, and is covered with numerous long golden-yellow rhizoids. It never seems to reach the light, and is quite destitute of chlorophyll. The largest specimens found are shown in figs. 5, 6, and 8, being 18 mm. and 13 mm. respectively in total length, and the smallest in figs. 11 and 12, these being from 1 mm. to 2mm. long. In its unbranched form the prothallus is carrot-shaped, tapering down gradually from a fairly thick head and upper region towards the basal first-formed end, which culminates in a more or less long-drawn-out point (figs. 1, 2, and 11). The first-formed basal region does not show such a marked primary tubercle as is so well known in the case of the prothalli of Lycopodium cernuum or in those of Ophioglossum and Helminthostachys, but there is commonly a succession of gentle swellings from, the original point of growth upwards by which the prothallus grows in girth (figs. 1, 2, 11, and 13). The actual head is generally the stoutest region (figs. 12, 13, & c.), being sometimes curiously swollen, and the growing apex is bluntly rounded.
Sooner or later the head of the prothallus forks dichotomously, and one of the branches so formed may later fork again. In some cases the first branching is postponed till after the prothallus has attained a length of as much as 8–10 mm. (figs. 1, 2), and the result is the carrot form; more often, however, the first forking takes place comparatively early (fig. 6), and many adult prothalli were found in which one of these
branches had developed into the main prothallus-body, whilst the other had either broken away or persisted towards the base of the first in a state of arrested growth (figs. 4, 5, 6). The forking generally seems to result at first in two equal apices of growth (figs. 1, 2), and hence may be termed dichotomous, and, except in the case of the first branching as just described, which takes place when the prothallus is still comparatively small, the resultant branches become more or less equally developed (figs. 2 and 5). Hance in most adult prothalli found the original simple carrot shape form had been lost, and the prothallus had become more irregular in appearance, such as is generally the case with epiphytic prothalli. Thus in this respect the prothalli of Tmesipteris can be compared with those of the epiphytic species of Lycopodium and Ophioglossum. In a few instances, moreover, such as those illustrated in figs. 6 and 66, a still greater irregularity of form had been brought about through the branching not taking place dichotomously. In the former of these two prothalli the forking seems to have been trichotomous. Still another irregularity in the form of adult prothalli is brought about by the equal development of both daughter branches at the first forking of the prothallus, not, as is usually the case, at an angle to one another, but in directions diametrically opposite (fig. 7). This is still more pronounced in the case of the large prothallus shown in fig. 8, in which one of the branches resulting from the first forking had forked again, the two branches of this second forking proceeding to develop in opposite directions to one another in the sme straight line. Thus the branched form of the adult prothallus is attained normally by the dichotomous forking of the apex, but I observed also a few instances in which short undeveloped branches had arisen apparently laterally. However, even in the most irregularly shaped adult individuals the manner of growth can always be easily traced, for even if the original long-drawn-out point be not preserved, yet the oldest region can always be distinguished from the rest of the prothallus by its darker brown or even almost black colour.
Fig. 1.—Complete prothallus, carrot form, bearing young plant, and showing original end intact. × 10.
Fig. 1A.—Original end of prothallus shown in fig, 1. × 24.
On some of the prothalli a large cup-shaped prominence with an obviously lacerated rim was to be seen (figs. 4 and 73). This is where a young plantlet had been broken away, the cup-shaped prominence having been formed by the localized outward growth of the prothallial tissues around the embryo and their final rupture by the developing plantlet. Such a point of attachment of the plant to its parent prothallus
may be seen sometimes in the lower regions of the latter (fig. 5), indicating that the growth of the prothallus is by no means arrested by the development on it of a plant, but may go on after the latter has attained. a considerable size or has even become detached from the prothallus.
Fig. 2.—Complete prothallus, carrot form, commencing to fork, bearing young plant. × 9.
Fig. 3.—Prothallus, carrot form, original end broken off, showing swollen head. × 12.
Fig. 4.—Prothallus, branched, one branch broken off, shows original end intact, also point of attachment of young plant. × 12.
Fig. 4A.—Original end of prothallus shown in fig. 4. × 36.
When first seen amongst the tangle of black aerial rootlets of the treefern stem the prothalli may easily be mistaken for broken portions of the rhizome of young plants or for very young complete isolated plantlets, and vice versa. Both the prothalli and the rhizomes are brown in colour, and both are covered fairly thickly with the long yellow-brown rhizoids or with the characteristic small brown circles formed by the persisting bases of broken-off rhizoids. The similarity holds also with regard to their growing apices, which are always somewhat swollen and are clear and whitish in appearance and show rhizoids only in their earlier stages of development. Each object dissected out has generally to be separately cleaned and examined under a low power of the microscope before its nature can be definitely determined. This is especially so in the case of the branched prothalli, whereas the carrot — shaped individuals are more easily recognized. However, generally speaking, the colour of the
prothallus is more opaquely brown than that of the rhizome, the latter appearing a clearer golden brown, with its surface cells outlined with great distinctness, this difference in appearance being due possibly to the denser fungal element in the interior tissues of the prothallus. The older basal regions of the prothalli are often darkly brown in colour or even blackish, owing not so much to any withering-away of the tissues as to the presence of the mycorhiza in this region in the cells immediately underlying the epidermis, and, in the oldest regions of all, in the epidermal cells also, as well as in those more centrally situated.
Fig. 5.—Complete branched prothallus of large size, bearing young plant which shows both rhizome and serial stem. × 3.
Fig. 6.—Complete branched prothallus of large size, one main branch showing further irregular branching. × 10
The prothallus in transverse section is round in outline (figs. 16 and 17), this being so throughout its length, so that its construction is consistently radial. Its growth in length is referable to the activity of a single cell (figs. 20 and 21), such as is the case also with the cylindrical prothalli of the Ophioglossaceae. A transverse section through the main
body of the prothallus shows its tissues to be composed of cells of uniform size and shape, there being no differentiation of central long conducting cells or of fungal zones such as are so well known in most of the types of Lycopodium prothalli. The dense fungal coils occupy uniformly practically all the cells in the central region, the epidermis and a zone three or four cells in width immediately underlying it alone being free from these coils. In the limbs of the larger prothalli this subepidermal layer sometimes contains much starch. Moreover, meristematic activity sometimes shows itself in these cells (fig. 19), though whether in connection with the storage of starch or with the development of the sexual organs is not quite clear. The mycorhiza extends uniformly right up through the length of the prothallus to close behind the actual apex, keeping pace with the forward growth of the latter. A series of transverse sections behind a growing apex shows that at its uppermost limit the mycorhiza occupies only a narrow central core of cells, which gradually tapers off upwards, and that in these cells the hyphae are more scantily developed. The fungal hyphae in these growing regions of the prothallus are wholly absent from the cells which surround the central core, this fact showing that when once the mycorhiza has entered the prothallus in its earliest stages of development no further infection is needed, but that the fungus extends upwards in a uniform manner, keeping pace with the growth of the prothallus. The clear white colour of the actual apex is, of course, due to the absence of the fungus from its cells. In the older parts of the prothallus hyphae can often be distinguished penetrating through the length of rhizoids and across the outer layers of cortical cells, but it is probable (as is also considered to be the case in other pteridophytic prothalli which are infected with a mycorhiza) that this signifies no organic connection between the fungal
Fig. 7.—Complete branched prothallus, in which the branches are not inclined to each other at angle but in opposite directions. × 10.
Fig. 8.—Branched prothallus, one branch broken; the other has branched again in the manner described for fig. 7. × 5.
hyphae within the prothallus and those in the surrounding humus. A great outward growth of hyphae was noticed from the surface of teased-up portions of young rhizomes which had been kept for some days in water in a watch-glass, and many of the threads showed what seemed to be single round spores at regular distances along their length. At its uppermost limit the hyphae of the mycorhiza in the interior cells of the prothallus are scantily developed, but farther back the coils become more dense. Throughout the greater part of the prothallus the fungal contents of each cell show as a dense globular mass,in which the identity of the hyphal threads can no longer be traced. These globular contents of the cells present a very characteristic feature both in the prothallus and young rhizome. (See Plate II.)
Fig. 9.—Old withered prothallus, carrot form, attached to plantlet which is broken above and below. × 3.
Fig. 10.—Old withered branched prothallus, attached to plantlet from which aerial stem is broken off. × 2.
Fig. 11.—Very young complete prothallus, showing original end intact and antheridia on its head. ×45.
Not a few well-grown prothalli showed the original point of growth almost intact, and the remains of the first-formed filament, which arises, presumably, immediately from the spore, could be very clearly seen (figs. 1A, 4A, 11, 12, and 13). In two instances—namely, the very young prothallus shown in fig. 11 and the much older one in fig. 1A — there was present at the extremity of the basal end a short filament of cells, two or three in length, which in the former case was seen to be incomplete,
but in the latter was apparently quite complete. The prothallus shown in fig. 4A tapered off at the basal end to a single cell, which showed no sign of original farther extension such as would compare with the longer filament in figs. 1A and 11. But the single cell in which the basal point of most of the youngest prothalli found by me terminated did give evidence of having had a farther cellular extension broken away from it. In all these prothalli the terminal basal cells, whether single or in the form of a short linear filament, all contained the same dense masses of the fungal element which are present in the other parts of the prothallus. Thus it would seem that the fungus enters the prothallus immediately the spore begins to germinate, unless perhaps we take it that it spreads downwards into the filament subsequent to the infection of the prothallus through the first-formed rhizoids. Probably the delicate original basal filament owes its preservation to the fact of the presence in its cells of these fungal masses, the collapse of the cells being thus prevented. At any rate, the preservation of the actual original point of the prothallus of Tmesipteris in so many individuals, some of which were well grown, is rather remarkable. It would seem, then, though it must be stated that the remains of the originating spore itself have not been seen, that on germination the spore gives rise to a short linear filament of cells, and that this, after from one to three or more single cells have been cut off, proceeds to the formation of a cell-mass. This basal primary tubercle is well preserved in the prothalli shown in figs. 1, 2, 4, 11, 12, and 13, and it will be seen that in most cases it shows no great development. The further stages of growth of the prothallus can be clearly seen from a comparison of the young and the older individuals shown in these figures. The prothallus grows in a succession of gentle swellings, each a little bigger than the last, the increased cell-multiplication which these swellings indicate being due probably to the accumulation of food material at the apex, consequent on the activity of the mycorhiza. In fig. 14 is shown one of the limbs of the large prothallus illustrated in fig. 6; serial sections through this limb showed that the cells of the apical region were packed with starch. Thus, as the prothallus grows, its apex becomes more and more bulky, so that the whole prothallus-body acquires the carrot form, until at length, owing probably to the stimulation set up by the presence of abundant
food contents in its cells, the head of the prothallus forks and the carrot form gives place to the branched form characteristic of the full-grown individuals.