276 Transactions. 



and the L. Selago types. It will be remembered that Thomas (16, p. 287) 

 draws attention to the variation in the length of the shaft in the pro- 

 thallus of Phylloglosmm. Miss Edgerley (8, p. 107) in her description of 

 the prothallus of L. Billardieri has noted that it is simpler in organiza- 

 tion than that of L. Phlegmaria, in that the elongated central cells in the 

 branches do not show the presence of pits on the walls, and that the 

 Ijaraphyses are only three cells in length and are never branched. The 

 prothalli of L. fastigiafum and of L. voluhile are in their mature form 

 very much alike, being flattish and saucer-like; but, as has been de- 

 scribed above, this form has been arrived at differently in the two cases. 

 The branching of the prothallus of L. fastigiatum would seem to be 

 I'eminiscent of the branching which is found in the L. Selago type, and 

 also in the epiphytic and the L. cemuum types. 



The close relationship between the form of the Lycopodium pro- 

 thallus and its habitat, as illustrated in the species dealt with in this 

 paper, must also be briefly noticed. The prothalli of L. cemuum, 

 L. ramnlosum, and L. laterale are delicate, minute, and short-lived, and 

 are surface-growing. The variation in length of the shaft of their pro- 

 thalli is probably to be put in connection with the varying depth at 

 which the spores germinate below the surface of the ground. These 

 three species occur in wet habitats where the establishment of the young 

 plant on the surface of the ground can readily take place. Ihe epiphytic 

 species of prothallus has an extensively branched and ramifying form. 

 Its typical habitat is a loose matted humus. The greatest variety is to be 

 found amongst those terrestrial species whose prothalli are subterranean. 

 Some of these subterranean prothalli, such as those of L. voluhile and 

 L. fastigiatum, occur chiefly in the loose humus in the top layer of soil 

 immediately beneath the overlying moss and vegetable growth. Tliese 

 are the prothalli whose form is flat and saucer-like owing to the super- 

 position of a horizontal mode of growth on the original vertical radial 

 habit. The prothallus of L. scariosum, on the other hand, occurs as a 

 rule much deeper and in more compact soil, and here the upright manner 

 of growth is preserved and the saucer-like depression on the upper surface 

 is absent. That view which would look upon the Lycopodium prothallus 

 as a very plastic one would seem to be more in accordance with the 

 foregoing observations thaci that other view which would regard the 

 different types of iDrothallus as having been genetically distinct from a 

 very remote period. 



IV. The Morphology of the Young Plant. 

 L. Billardieri. 



The young plant takes its origin from the central mass of tissue of 

 the prothallus. It first appears as a simple cylindrical stem (figs. 7 

 and 9), whicli it quite destitute of leaves for a length of 1-6 cm., accord- 

 ing to the depth at which the prothallus is buried in the humus. The 

 first-formed one or two leaves are generally scale-like, but the succeeding 

 ones are large and of the mature form (figs. 10-12). A longitudinal 

 section of the prothallus and young plant shown in fig. 7 revealed the 

 fact that the latter possessed a fairly large firm foot, and that there 

 was a distinct epithelial layer of cells where the latter came in contact 

 with the prothallial tissues (fig. 8). It also showed the "first root" 

 (throughout this paper this term is used to signify the first functional 

 root) making its appearance as a protuberance at the base of the stem. 



