HoLLOWAY. — Sti/rJies in the Xeiv Zcnhind Species of Lycopodiiim. 203 



moss itself, where they show no adhering earthy matter at all, or even, 

 as in the case of the former species, growing on the surface of the soil with 

 the upper region of the prothallus exposed to the light, being then in their 

 upper region a vivid green in colour. In Part I of these Studies (16, p. 262) 

 I noted that the prothalli of L. scariosum often lie at a depth of 8 cm. to 

 10 cm. I have not been able to find in Bruchmann's papers any reference 

 to the comparative depth at which he found the prothalli of the three 

 European species. I would suggest that the two different types are an 

 expression of the fact that the species of prothalli which conform to them 

 grow respectively in a shallower and in a deeper stratum of soil, the deeper- 

 growing prothallus having more thoroughly departed from the self-nourishing 

 chorophyll habit. The fact that there are two such distinct types of 

 prothallus in the Clavata section, and that along with these there go two 

 distinct types of heterophylly, may perhaps show that more than one 

 distinct strain of evolution is included in this section. 



The Young Plant. 



HeteropJujUij. — The development of heterophylly in the seedling of 

 L. volubile and L. scario.nmi opens up the question as to what is the nature 

 of the stimulus which calls forth this character. That it is an extreme 

 instance of adaptation is beyond doubt. Heterophylly is present in the 

 genus Selaginella, but it would be difficult to take the view that in the 

 Lycopodiuni genus it is a phylogenetic character. Thus modern systematists 

 have abolished Baker's subgenus Diphasium, in which he groups together 

 the heterophyllous species, and have distributed these species according to 

 their more natural affinities. Heterophylly is an adaptation which has 

 appeared also in the section Inundata. 



What has been the particular external stimulus to call forth this cha- 

 racter ? It is to be noted that heterophylly is not found in any species 

 whose main stem is subterranean, and whose lateral branches accordingly 

 emerge from the ground in a stiffly erect and tree-like form. L. clensum and 

 L. fastigiatum possess this latter habit. Their erect, tree-like aerial branches 

 are obviously stimulated by the light in no one direction more than in any 

 other. Certainly the aerial branches of L. scariosum fre([uently grow erect in 

 the lowlands of Westland, as described above, and still possess the charac- 

 teristic heterophylly, but this is simply on account of the luxuriant massed 

 development of the plants in these localities. And, again, the species 

 L. Jussiaei Desv. of South America, which is joined by both Pritzel and Baker 

 with L. scariosum, shows more or less erect-growing branches along with the 

 heterophyllous habit. But in both these species the flattened nature of the 

 branching is preserved even in the more erect-growing branches, the case being 

 very different with the dendroid, fastigiately branched shoots of L. clensum 

 and L. fastigiatum. Heterophylly, then, is an adaptation which goes hand 

 in hand with the more or less flattened habit of the branches in which thev 

 are somewhat spread out in the plane of the surface over which they are 

 growing. The particular stimulus to the development of the large leaves 

 and to their dorsiventral arrangement is probably that simply of the light, 

 which, falling more or less at right angles upon the branches, stimulates 

 the leaves, which are tiny and needle-shaped and catch very little sunlight, 

 to place themselves in such a position and to acquire such a form that they 

 can intercept the maximum amount. In both L. volubile and L. scariosum 

 the large-shaped leaves are not naturally borne in the lateral position 

 but adapt themselves to it. In the case of L. volubile two neighbouring 



