EMBRYOLOGY 



345 



developed in more open soil suggests the origin of the phlegmaria-type : 

 while its green sub-aerial forms are reminiscent of the cernuum-type. A 

 plant which shows such plasticity is clearly not far removed from the self- 

 nourishing condition of the prothallus, which was probably the primitive 

 condition for them all. 



These remarks upon the curiously divergent development of the pro- 

 thallus in the genus Lycopodium are a necessary preliminary to the study of 

 the embryogeny in the genus ; for it is impossible to understand the 

 comparisons of the different forms of embryo without some knowledge of 

 the prothalli which produce them. In all the species of Lycopodium in which 

 the embryogeny is accurately known, an early stage of the embryo is 

 found in which it consists of a suspensor, and of two tiers, each composed 

 of four cells (Fig. 182). The first cleavages are variable in their succession,, 

 as is found to be the case also in other embryos ; 

 but their position shows considerable constancy. 

 It is stated that from the lower tier of cells, 

 i.e. that adjoining the suspensor, the structure 

 designated the foot arises, while the upper tier 

 gives origin to all the other parts of the 

 embryo, and the correctness of the statement 

 is borne out by numerous drawings. But after 

 the first stages are past there is usually no 

 sharp limit between the tissue composing the 

 foot and that of the other parts : in the simplest 

 cases it appears as though the foot were merely 

 a region of tissue lying between the suspensor 

 and the upper tier, rather than a definite organ 

 or part. Functionally, the foot does not appear 



to be differentiated from the suspensor in the genus Lycopodium, and it 

 shares with it the office of maintaining connection with the prothallus. Not- 

 withstanding the initial similarity which thus rules in the embryos of the 

 genus, the further steps of the embryogeny differ according to the different 

 forms of prothallus above described ; and it becomes a question which of 

 the divergent types is to be held as the most nearly reflecting the original 

 condition, and which as' later and derivative. 



The type of L. Selago may be taken first, since it does not show any 

 high degree of specialisation in its variable gametophyte, while it has 

 been seen above that its mature sporophyte is one of the least differen- 

 tiated in the genus. Its early embryogeny, so far as is known, conforms 

 to the usual type, as above stated. The foot originates from the lower 

 tier, and the various parts of the embryo from the upper. 1 But the foot 

 is only slightly developed. The upper tier of cells soon assumes a green 

 colour and unsymmetrical form, owing to the lateral upgrowth of the first 

 leaf or cotyledon, while the apex of the axis also originates early, near 



1 Bruchmann, I.e., pp. 97-103. 



FIG. 182. 



Diagram illustrating the primary 

 segmentation of the zygote in Lyco- 

 podium. /, I first segmentation 

 wall which separates the suspensor, 

 here cross-hatched, b, b separates 

 a lower tier (foot-tier) here dotted, 

 from an upper tier (stem-tier) left 

 clear ; each tier consists at first of 

 four cells. The wall b, b corresponds 

 to the wall IV. -IV. in Figs. 183, 186, 

 and to wall II.-II. in Fig. 185. 



