EMBRYOLOGY 



347 



substantially the same : the hypocotyl becomes elongated as the seedling 



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



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



the scheme (Fig. 182), and the lower tier, as in L. 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 L. Phlegmaria 



with Bruchmann's less complete 



series for L. 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 clavati(m-annotiniim-\y\)Z 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 aHove 

 (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. 179 B with Fig. i86c). The 





Prothalli of 



plants " 



