Art. IX.—Preliminary Note on the Protocorm of Lycopodium laterale R. Br. Prodr.
[Read before the Philosophical Institute of Canterbury, 2nd December, 1914.]
It is well known that in the embryo plant of Lycopodium cernuum, and also in those of L. inundatum and L. salakense, the development of the tuberous organ termed the “protocorm” is an important phase in the ontogeny, bridging over the period between the early stage in which the embryo plant is wholly dependent upon its parent prothallus, and its subsequent development, in which it has obtained independence through the establishment of a root-system. Moreover, in the related genus, Phylloglossum, a protocorm is present, and this plant has been spoken of as “a permanently embryonic form of Lycopod,” for the protocorm is there not a temporary organ, but the plant-body proper.
The Lycopod protocorm is the subject of widely differing interpretations. The first of these, and one which invests this organ with considerable interest and importance, is that it is a highly primitive organ, and represents an ancestral phase in the evolution of the free-living Lycopodium sporophyte, and perhaps also in the evolution of vascular plants generally (Treub). A second interpretation is that the protocorm is not to be regarded so much a primitive organ as an opportunist growth, and that, even if it does play an important part in the establishment of certain Lycopod embryos, it is not to be regarded as representing a phylogenetic feature in the Lycopodiaceae, and still less in vascular plants as a whole (Bower). A third interpretation has more recently been put forward, which, taking into account the great development of the stem in Palaeozoic Lycopods, and being unwilling to regard the protocorm either as a highly primitive organ or as a mere parenchymatous swelling, would look upon it as a modified form of stem due to reduction (Brown, “New Phytologist,” vol. 12, p. 222, June, 1913).
In view, therefore, of the puzzling nature of this organ, it is interesting to note that it has been found in yet another species of Lycopodium—viz., L. laterale—and that it there assumes a much greater size, and plays a much more important part in the establishment of the young plant than in the other species of Lycopodium in which it has been recorded.
In a paper entitled “A Comparative Study of the Anatomy of Six New Zealand Species of Lycopodium,” published in the Transactions of the New Zealand Institute, vol. 42, 1910, the writer gave a short account of the protocorm of L. laterale, with five small figures (figs. 5–9, pl. xxxi). No definite conclusion was there reached as to the interpretation of the protocorm. The present paper is a summary of the writer's further observations upon this organ.
The prothallus of L. laterale is of the cernuum type, as is the case also for the prothalli of each of the other species in which a protocorm has been found, excluding the possibly doubtful case of L. phlegmaria. At the stage at which the young plant of L. laterale consists of a basal protocorm surrounded by two or three protophylls it is similar in appearance to the young plant of L. cernuum. The succeeding stages in its development are, however, noteworthy. The protocorm grows sideways, owing to the lateral development of two new protophylls, whose swollen
bases, in the majority of young plants of this age that were examined, show as a tuberous region separated from the first-formed protocorm proper by a more or less marked constriction. In L. cernuum the protophylls are all normally developed on the upper side of the small tuberous protocorm, and not as a lateral extension of it. The writer has, however, out of a large number of young plants of L. cernuum that he has examined, observed that the protocorm in one or two instances showed a distinct tendency to grow laterally, one young plant showing seven protophylls on such an extended protocorm. In older plants of L. laterale it was observed that the original protocorm could often be still distinguished from the later-formed protocormous extension, and that the former easily broke away from the latter in the process of cleaning.
The further growth of the young plant takes place by the continued lateral development of protophylls, and the consequent extension of the protocorm by their swollen bases, the protocorm increasing in thickness the farther from the original end. The protophylls are more or less arranged in two rows on its dorsal surface, this arrangement indicating that their development, as also that of the rhizome itself, has taken place very regularly. The protocormous rhizome thus formed bears ventrally a mat of rhizoids. It gradually loses its green colour and semitranslucent appearance, and in the fully developed stage has become yellowish and opaque and firm, whilst after the differentiation of the stem-axis both the protophylls and the rhizoids decay away. The rhizome consists of parenchymatous tissue throughout, the cells of the central region being smaller and more compact, whilst those nearer the surface are larger, and show air-spaces. A strand of vascular tissue is present in each protophyll, and passes down into the body of the rhizome, where it ends blindly. At the stage at which a stem-apex is differentiated there are from eight to twelve protophylls, and the rhizome is from 3mm. to 5 mm. in length and from 1 mm. to 2 mm. in thickness. In one instance it was observed that the rhizome had forked into two equal branches, and that on each of these a stem was developing.
The stem-axis arises at some point on the dorsal surface of the rhizome towards its growing end, or even almost at the end itself, and is indicated by the aggregation of protophylls. At the same time, vascular tissues are initiated from the stem-apex and extend down into the body of the rhizome, receiving on the way strands from the neighbouring protophylls. In the rhizome these vascular tissues bend round at a sharp angle, and, surrounded by a slight zone of sclerenchyma, take a course through the body of the rhizome, though nearer its dorsal surface, towards its growing end. The latter at the same time grows outwards and downwards to form a finger-like protuberance into which the vascular tissues pass. Thus the extension in length of the protocormous rhizome is brought to a close by the initiation of this exogenously developed first root. The writer has observed that in the young plant of L. cernuum the vascular tissues of the stem behave in the same way as in L. laterale. They lead down bodily into the upper region of the protocorm, and thence decline towards the finger-like protuberance of the rhizome, which is the first root, though in this species the angle of declination is more gradual than in L. laterale. The course of the main vascular strand in its relation to the protocorm just described may be compared with what obtains in the young plant of L. clavatum and other species, where the vascular system does not extend into the tissues of the large intraprothallial foot. All subsequently formed roots in the young plants of both L. laterale and L. cernuum emerge
adventitiously from the stem, and do not pass through the tissues of the protocorm. The protocormous rhizome of L. laterale is a persistent organ, and may be recognized in plants 2 in. or even more in height. It may be stated that the protophylls both on the protocorm proper and on the rhizome are essentially similar in appearance and structure to the ordinary vegetative leaves borne on the young stem-axis. In no case either in L. laterale or in L. cernuum was any transition between them to be observed.
L. laterale grows on damp peaty ground and around the margins of marshes. L. cernuum and Phylloglossum also occur in New Zealand in the Auckland Province in much the same kind of habitat, though on higher ground rather than in the hollows. During the greater part of the year ground of this nature is continuously wet, and holds much water, but in the summer months it is liable to be dried up. The view taken in this paper is that the large development of the protocorm in L. laterale is an adaptation to carry the young plant over the dry season. This would seem to be indicated in the continued lateral development of protophylls with swollen bases, and by the distinction so frequently to be observed between the original protocorm and the protocormous rhizome. The fact that the protocorm of L. cernuum also occasionally extends laterally as a rhizome indicates that the Lycopod protocorm is a plastic organ, and that too much stress must not be laid from a phylogenetic point of view upon the fact of its normally large development in L. laterale. The writer desires rather to emphasize the fact that a well-developed protocorm, which in its first stage is of the same nature as that so well known in L. cernuum, has been found also in L. laterale; that it is there correlated with a prothallus of the cernuum type; and that L. laterale belongs to the same subgenus Rhopalostachya as do the other members of the Lycopodiaceae (with the possible exception of L. phlegmaria) in which a protocorm has been recorded.
1. In L. laterale and occasionally also in L. cernuum (though there to a less extent) the protocorm is capable of considerable development, and constitutes the plant-body proper for a lengthy period. The vascular strand of the stem and first root takes a course through its tissues. The rhizome of L. laterale may even branch and give rise to more than one stem-axis.
2. There is a marked developmental distinction in L. laterale between the original protocorm and its rhizomatous extension. This suggests that the two portions must be interpreted apart from one another.
3. In L. laterale the manner of development of the protocormous rhizome suggests that its large size is an adaptation to carry the young plant over the dry season which normally always follows the wet winter season.
4. In L. laterale the protocorm is associated with the cernuum type of prothallus, and this is the case also in the other Lycopodiaceae in which a protocorm has been recorded. This type of prothallus has been stated on other grounds to be primitive for the genus. The fact that these protocormous species belong to two groups in the subgenus Rhopalostachya suggests a certain degree of antiquity for the protocorm within the genus Lycopodium; and, assuming the primitive nature of the cernuum type of prothallus, would also suggest that the subgenus Rhopalostachya comprises the more primitive members of the Lycopodiaceae, and that the genus as a whole should be read as a reduction series rather than as a series which has progressed from those forms which show the simpler type of sporophyte.