THEORY OF THE PROTOCORM 



223 



The body of the embryo thus exposed bears the cotyledon, and a variable 

 number of leaves directed upwards, but it terminates downwards in a tuber- 

 like body provided with rhizoids (compare Fig. 21, p. 37). At first there 

 is no root, and in extreme examples the appearance of the first root may 

 be deferred for a considerable time ; but so soon as the normal aerial 

 shoot with leaves is defined, the first root soon penetrates the soil, and 



FIG. 115. 



Young embryo of Lycopodiitm cernuutn, beginning to project from the prothallus. 

 ar=archegonium ; s = suspensor ; cot = cotyledon ; ta = embryonic tubercle. X 300. 

 (After Treiib.) 



establishes the plant in the usual way. The tuber which thus precedes 

 the establishment of the plant by means of a root was called by 

 Treub the "protocorm," and he regarded it as a rudimentary structure, 

 which was the phyletic forerunner of the leafy shoot as now seen 

 generally in Pteridophytes. It is represented, however, only in few cases, 

 and is not constant even in the genus Lycopodium : for instance, it 

 is absent in L. Se/ago, and also in L. Phlegmaria and L. clavatum. 

 In PhylloglossuiHi on the other hand, this type of development is not 

 only found in the primary embryology, but is repeated constantly in 



