106 EMBRYOGENESIS IN PLANTS 



L. demum and L. billardieri, in which the foot is greatly distended, and 

 by the protocormous species L. cennium, L. laterale, and L. ramulosum, 

 which develop at the soil surface, and which, as well as being supplied 

 with carbohydrate from substantial prothalli, are themselves green and 

 photosynthetically active from an early stage. That the carbohydrate 

 supply in the latter three species far exceeds growth requirements is 

 shown by the development of a conspicuous, starch-filled, tuberous 

 protocorm. With this development goes a suppression or delay in the 

 activity of the shoot apex. The relation of these developments to light 

 and other factors would be an investigation of considerable interest 

 and importance. These observations on carbohydrate metabolism in 

 the embryogeny can be extended to characters seen in the adult plant. 

 Thus, some species of Lycopodium have a large sappy cortex with 

 abundant starch, whereas others have little starch but a conspicuous 

 development of fibrous tissue. Both environmental and genetical factors 

 are involved. On the evidence thus far available, those species with a 

 protocorm are also conspicuous for starch deposition in the adult 

 state. 



Evolutionary Relationships. In the fight of the data now presented, 

 can it be said with any assurance that one type of embryogeny, e.g. that 

 of L. selago, is more primitive than another, e.g. that of L. clavatum or 

 L. cernuiiml As we have seen, there are distmct differences and these 

 can be ascribed to factors in the genetic constitution and in the environ- 

 ment. But as to the relative evolutionary status of one type as compared 

 with another, it seems almost impossible to arrive at a definite conclu- 

 sion. If, for the sake of argument, we assume that L. selago is a central, 

 primitive type, then the condition which we see in L. clavatum or L. 

 cernuum could be due to mutation or chromosomal change, possibly 

 affecting protein metabolism. In this case, these species would be 

 'derivative' but not necessarily upgrade forms. In a cytological investi- 

 gation of the five British species of Lycopodium, Manton (1950) has 

 reached the conclusion that these are so very unlike cytologically that 

 they have probably been taxonomically widely divergent for a very long 

 period of time ; and L. selago, so often considered to be a central and 

 primitive type, is found, on the cytological evidence, to be a hybrid of 

 very high chromosome number. 



