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 
Fic. 115. 
Young embryo of Lycopodium cernuunt, beginning to project from the prothallus. 
a@r=archegonium; s=suspensor; cof=cotyledon; z¢#b=embryonic tubercle. 300. 
(After Treub.) 
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 Z. Se/ago, and also in ZL. Phlegmaria and L. clavatum. 
In Phylloglossum, on the other hand, this type of development is not 
only found in the primary embryology, but is repeated constantly in 
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