142 Elementary Biology. 



into a portion known as the suspensor and the embryo 

 proper. The suspensor forms the upper portion of the pro- 

 embryo, that is, the part next the canal cells. The suspensor 

 by increase in length pushes the embryo proper deep into 

 the endosperm or vegetative part of the thallus. There the 

 mapping out of the embryo into cotyledon (or seed-leaf), 

 stem, and root takes place before its independent existence 

 outside the spore-wall is established. During the embryonic 

 period, as already explained, the thallus acts as nourishment 

 to the young plant. 



There are many points of resemblance between the 

 structure and life-history of Selaginella and the Gymno- 

 spennce (pine group), which point to their being transition 

 forms between the Selaginellida and the Angiosperma. 

 There is no difficulty in tracing the relationship between 

 Selaginella and such a type as Pinus, notwithstanding the 

 tremendous difference in size ; for in past ages of the earth's 

 history closely allied forms have flourished which were even 

 more gigantic than the largest pines, the spores of which 

 form no small part of the coal of ordinary use. We have 

 already drawn attention to the likeness between the cones 

 of the two forms, and the structure of the sporangia also 

 shows no great morphological differences. The anatomy of 

 the stem, however, is much closer in its nature to that of the 

 flowering plant than to that of Selaginella ; but beyond this 

 general statement further exposition of their agreements and 

 differences must be omitted here. 



Lastly, and by way of summary, we must emphasise the 

 fact that we have in Selaginella true alternation of generations 

 just as in the fern, only here the sexual generation is para- 

 sitic on, or at least commensal with, the asexual. If we were 

 to attempt to represent this relationship diagram matically we 

 might do so thus : 



""";st"(J i X ST J 



AST< + ; : AST< + 



\8 T 2 H X S T ? 



-- ! 



