EMBRYOLOGY 



347 





substantially the same : the hypocotyl becomes elongated as the seedling 

 develops, and the whole appearance of the seedling resembles that of 

 Z. Selago. The primary segmentation in L. Phlegmaria is according to 

 the scheme (Fig. 182), and the lower tier, as in Z. Selago, forms only the 

 foot, which attains no great size (Fig. 185). The upper tier develops 

 unsymmetrically from the first, the side which will form the first leaf 

 growing more strongly; close to the base of the cotyledon, and apparently 

 lateral owing to the stronger growth of the latter, but in reality terminal, 

 arises the apex of the axis (T, Figs. 185 A, B) ; it is, in fact, initiated in 

 close proximity to the organic centre of the upper tier. The root 

 (R, Fig. 185 B) also originates from 

 the upper tier. A comparison of 

 Treub's drawings of Z. Phlegmaria 

 with Bruchmann's less complete 

 series for Z. Selago shows clearly the 

 substantial similarity of the embryo- 

 geny in the two species. It will 

 be remembered that the flattened 

 prothalli of the latter species, formed 

 near the level of the soil, have been 

 held to prefigure the strap-shaped 

 sexual branches of the Phlegmaria 

 prothallus, though the latter shows 

 its higher specialisation for a sa- 

 prophytic habit in its filamentous 

 development and in its frequent 

 branching. On the other hand, as 

 regards the sporophyte, it has been 

 seen that the Phlegmaria type is not 

 one of the highest developed, but is 

 associated with Selago in the sub- 

 genus Urostachya. This being so, it is natural to find their embryos so 

 similar, notwithstanding the difference in specialisation of the prothalli 

 themselves. Lastly, both embryos are from the first subaerial : their first 

 leaves are green assimilating organs, and differ in no essential degree from 

 the normal foliage leaves. This may probably be held to be a primitive 

 condition. 



But in the davatum-annotinum-ty^e the case is different. It has been 

 seen that there the prothallus is developed underground, often at a considerable 

 depth, and this brings with it modifications of the embryogeny. The first 

 steps in the development are the same as in the types described above 

 (Fig. 1 86 A); but very soon there is a conspicuous enlargement of the 

 tissue of the foot, derived from the lower tier, adjoining the suspensor 

 (Fig- i86B): a large spherical swelling is thus formed, which remains as 

 an intra-prothallial haustorium (compare Fig. 1796 with Fig. i86c). The 



FIG. 184. 



Prothalli of Lycopodiunt Selago with seedling plants 

 X 3. (After Bruchmann.) 



