In two previous papers (11, pp. 357–61, and 12) the writer has given an account of the young plant of this species. The chief points in its development will here be given in reference to several microphotographs and drawings which are included in this paper. The prothallus of this species has already been described as belonging to the L. cernuum type. During the early stages in its develop-
ment the young plant is very similar to that of L. cernuum. It consists of a basal tuberous protocorm surmounted by one or two protophylls, and is connected with its parent prothallus by a foot which can clearly be distinguished through the prothallial tissues (figs. 14—16). The protocorm and protophylls are a vivid green in colour, and stomata occur on the latter. The protocorm bears numerous long rhizoids. From this stage onwards the young plant of L. laterale differs in its development from what normally takes place
Figs. 54–56.—Lycopodium laterale. Young plants, showing beginning of extension of protocorm. × 16.
Fig. 57.—Lycopodium laterale. Young plant shown in Fig. 56; side view. × 16.
Fig. 58.—Lycopodium laterale. Young plant with four protophylls. × 16
Fig. 59.—Lycopodium laterale. Young plant shown in Fig. 58; the latest-formed protophylls in end view. × 16.
Fig. 60.—Lycopodium laterale Young plant shown in Fig. 58; underneath view. × 16.
in the case of L cernuum. The third protophyll arises in a lateral position (Fig. 54), and when it is full-grown its base shows as a swelling clearly to be distinguished from the original protocormous tuber (figs 55 and 56). In Fig 57 the young plant shown in Fig 56 is depicted in a sideways position, in which the two distinct swellings are clearly seen. The fourth protophyll arises alongside the third, and forms a pair with it (Fig. 58). Here again it is to be observed that the swollen bases of these two protophylls are distinct from one another. In Fig 59 are shown in end view the two latest-formed members of the plant illustrated in Fig 58 Fig 60 is an under view of the same plant, showing
the foot on the first-formed protocorm proper, and the bases of the two latest-formed protophylls. A transverse section of such a plant through the thickest region of the plant-body reveals clearly the original tuber and the later extension of it, but also shows that internally the swollen bases of the third and fourth protophylls are
Figs. 61, 62.—Lycopodium laterale. Transverse section of two such young plants as shown in Fig. 58. 61, × 60; 62, × 35.
completely fused, although externally they can be distinguished (figs. 61 and 62). In no case was the prothallus found still attached to a young plant which bore more than the two original protophylls. The plant-body continues to grow sideways owing to the lateral development of new protophylls (figs. 63–65). In the majority of the young developing plants that were examined it was observed that the first-formed protocorm proper could clearly be distinguished from the later-formed protocormous extension, there being a well-marked constriction between
the two parts (figs. 64 and 65). In the cleaning process the two portions in not a few cases broke away from each other. The protocormous rhizome continues to elongate laterally, owing to the further development of protophylls, till there are as many as eight to twelve of the latter. The protophylls arise in pairs, and in many cases they are arranged in two more or less distinct rows along the dorsal side of the rhizome. This, of course, indicates that their development, and so also that of the whole plant-body, has taken place very regularly. The rhizome is
covered on its ventral surface with a mat of rhizoids. In the earlier stages of its development it is bright green in colour and semi-translucent, but later it becomes yellowish and opaque and very firm. The
Fig. 66.—Lycopodium laterale. Young plant with fully developed protocorm and a very young stem-axis. × 4.
Fig. 67.—Lycopodium laterale. Young plant with a branched protocormous rhizome and two stem-axes. × 4.
Figs. 68, 69.—Lycopodium laterale. Young plant with young stem-axis and first root just showing. 68, × 4; 69, × 5.
total length of the fully developed rhizome is from 3 mm. to 5 mm., and its thickness from 1 mm to 2 mm. In one instance it was observed to have forked into two equal branches, and on each of these a young stem-axis was developing (Fig. 67).
Fig. 70.—Lycopodium laterale. Transverse section of fully grown protocormous rhizome before development of vascular strand, showing two protophylls above and groove on ventral side. × 30
Fig. 72.—Lycopodium laterale. Portion of rhizome shown in Fig. 71, showing relation of vascular strand to the tissues of the protocormous rhizome. × 225.
The rhizome consists of parenchymatous tissues throughout, the cells of the central region being smaller and more compact, while those nearer the surface are larger and sometimes show air-spaces at their angles (Fig. 70) The centrally placed cells stain much darker with haema-
toxylin than those nearer the surface, owing to the presence in them of fairly abundant cell-contents. Each protophyll possesses a simple vascular strand, which penetrates into the upper region of the rhizome,
Fig. 71.—Lycopodium laterale. Longitudinal section of fully grown protocormous rhizome, showing young stem-apex and course of vascular strand. × 25.
and there ends blindly (Fig. 70). A transverse section of a fully grown rhizome, such as is shown in Fig. 70, shows a well-marked groove running ventrally along the length of the rhizome. This is probably a consequence
Fig. 73.—Lycopodium laterale. Longitudinal section of fully grown protocormous rhizome, showing initiation of first root. × 25.
of the fact that the body of the rhizome is formed by the swollen bases of the two rows of its protophylls. The stem-axis is initiated eventually at some point of the dorsal surface, and is marked by the
aggregation of several protophylls. This takes place either close to the growing end of the rhizome or at some point farther away from it, although the position of the young stem is always nearer to the growing end than to the first-formed protocorm proper (figs. 66–69, 71, 73, 75–78). Almost immediately vascular tissues are initiated from the stem-apex, and extend down into the upper region of the rhizome, receiving on the way strands from the neighbouring protophylls (Fig. 71). In the rhizome these vascular tissues bend round at an angle which is more or less sharp according to whether the stem-apex is farther away from the growing-point of the rhizome (Fig. 71) or close to it (Fig. 73), and take a course through the tissues of the rhizome, near to its dorsal surface, towards its growing end. At a later stage this vascular strand is surrounded by a slight zone of sclerenchyma (Plate XVII, Fig. 1). The arrangement of the
Fig. 74—Lycopodium laterale. Portion of rhizome shown in Fig. 73, showing relation of vascular strand to the apex of root. × 225.
elements in the strand is described in the next section of this paper. Fig. 72 is a highly magnified drawing of a portion of Fig. 71, and shows that the vascular strand of the stem and first root as it passes through the body of the rhizome is developed from the actual tissues of the rhizome. The same fact is also apparent from Fig 74, which is a magnified drawing of a portion of Fig. 73, and from Plate XVII, figs. 1 and 2 At the same time that the vascular strand from the stem-apex is taking form along the body of the rhizome in the direction of its growing end, the latter at a point on the surface towards the dorsal side begins to grow outwards and downwards to form a finger-like protuberance (figs. 68, 69, 73, 74). Into this protuberance the vascular strand passes. This is the first root. The extension in length of the rhizome is brought to a close by its initiation. All subsequently formed roots in the young plant emerge adventitiously from the stem, and do not pass through the
Fig. 1.–Lycopodium laterale. Transverse section of protocormous rhizome, showing relation of vascular strand to tissues of rhizome, × 40. (N.B.—In this figure and in the other plate figures which accompany this paper the cell have all been carefully outlined on the photograph with indian ink, in order to ensure clear reproduction.)
Fig. 2.—Lycopodium laterale. Longitudinal section of portion of protocormous rhizome, showing course of vascular tissue from stem-apex to first rot × 35
Fig. 3.—Lycopodium laterale. Complete prothallus. × 30
tissues of the protocormous rhizome. The leaves on the young stem-axis are in nowise different from the protophylls on the protocorm proper or on the rhizome; in fact, the ordinary vegetative leaves of the adult plant also have much the same form. The protocormous rhizome of this species is a persistent organ, owing to its large size and firmness. It may be recognized at the base of the stem of young plants which are 2 in. or even more in height.