Early Stages in the Development.
I have been fortunate to secure several very young prothalli-of the two species L. laterale and L. ramulosum. No doubt the only perfectly satisfactory way of obtaining the youngest stages of growth in any prothallus is by the experimental germination of the spores in the laboratory under close observation. Even this method, however, is not above suspicion, for one can never be sure to what extent the artificial conditions influence the form of the young prothallus. Very young prothalli dissected out of soil collected in the field are open to the criticism that they may belong to some plant other than the one under consideration. I can only say that with regard to the young prothalli now to be described I am quite satisfied that they belong to the particular species of Lycopodium to which I have ascribed them, and the reasons will be stated.
In the case of L. cernuum, I have found no young stages. Treub has, however, described the germination of the spore in this species, having successfully carried this out in the laboratory. His figures, which I have
seen only in certain standard books of reference, show that a tubercle is at once initiated, which is apparently at first devoid of fungus. This tubercle was called by Treub the “primary tubercle,” and he supposed that it was a characteristic feature of the Lycopodium prothallus. He was also successful in experimentally cultivating the prothalli of L. salakense. This also begins with a globular body, the “primary tubercle,” from which several thin filamentous branches arise, one of the latter eventually thickening and producing the sexual organs. This prothallus remains altogether free from a symbiotic fungus. Treub also germinated the spores of L. curvatum Sw., from which he obtained “primary tubercles,” but he was unable to induce them to develop farther. All the mature prothalli of L. cernuum which I have found show the rounded basal tubercle, although in some instances this is somewhat pointed below (see 7, figs. 17–21, and figs. 38–44 in the present paper).
I have dissected out a number of young prothalli of the species L. ramulosum along with the more mature forms, and some of these are shown in figs. 61–65. I judge these to belong to this species since they are of the typical Lycopodium form and no other species of Lycopodium were present in the two localities from which they came. No other kind of prothallus was ever found by me in the turves from which I dissected out those of L. ramulosum, nor was any species of fern present in the near neighbourhood, except Gleichenia dicarpa. Finally, the individuals which are shown in figs. 61–65 form a series which leads on to the more mature prothalli which undoubtedly belong to L. ramulosum. The youngest prothallus was that shown in fig. 61. In it there was no basal swelling, nor was there any indication of the presence of a fungus. The lower half consisted of a single linear row of cells, whilst in the upper half there was a gradual increase in the number of cells and in the girth of the filament towards the apex. This upper half was not a flat expansion of cells as in the usual fern prothallus, but was radial in build. The cells at the apex were smaller than those farther back, and evidently functioned as the meristem. The whole filament contained chlorophyll corpuscles. The original spore-case was still attached to the basal cell. In fig. 62 is shown a prothallus in which the first-formed filamentous stage was very short, passing almost immediately and suddenly into a globular mass of cells which could be called the “primary tubercle.” This prothallus also was quite free of fungus, being green throughout. The apical region consisted of small-celled tissue, and a young sexual organ was developing near by. A group of rhizoids was borne on the basal tubercle. In the formation of a fungusless primary tubercle this young prothallus corresponds very closely to those early stages in L. cernuum, L. salakense, and L. curvatum described by Treub. With regard to the mature prothallus of L. ramulosum, the conclusion I arrived at was that any swelling in the fungal regions was primarily due to the localized presence of the fungus. From these other three species, however, it is apparent that there may be at first a primary tubercle quite apart from the presence of the fungus, and this also appears in such a young prothallus of L. ramulosum as that shown in fig. 62. However, this is not always the case, as in the particular prothallus shown in fig. 61. The three young prothalli shown in figs. 63–65 all possessed a primary tubercle, which was infested by fungus.
Are we to consider that a primary tubercle is a fundamental feature in the structure plan of the Lycopodium prothallus, or is it to be regarded as an added feature? Possibly we are to regard the radially-built filament
as being the original typical condition, this filament being drawn out, or short and bulky, according to whether growth has taken place rapidly or slowly. The basal tubercle of the mature prothallus when present would, according to this view, be the result largely of secondary cell-divisions
Fig. 61.—L. ramulosum. Very young prothallus, in general view, with spore still attached. × 75.
Fig. 62.—L. ramulosum. Young prothallus, in general view, showing basal tubercle with no fungus. × 45.
Figs. 63, 64.—L. ramulosum. Young prothalli, in general view, showing basal tubercle with fungus. × 30.
Fig. 65.—L. ramulosum. Young prothallus, in general view, showing first-formed lobes and the initiation of a second fungal region. × 45.
which had taken place in the first-formed basal cells owing to the stimulus excited by the storing of food material. Prothalli cultivated under artificial conditions seem generally to develop slowly, and this may explain why it is that none of Treub's prothalli showed a first-formed filamentous
stage. There can be no doubt that the symbiotic association of a fungus with the Lycopodium prothallus is a further added feature, the mature form of the Lycopodium prothalli being determined mainly by the nature of this association.
I found one very young prothallus of L. laterale which was entangled in the rhizoids and lobes of the large prothallus shown in fig. 48. This young prothallus is shown in fig. 49. It consists of a filament of cells which is green throughout, the filament being one cell in width in its lower half and two cells in width in its upper. At the apex there is a single small cell which is clearly functioning as the apical cell. In this species also, as in L. ramulosum, the primary tubercle is thus not invariably present.