168 



PLANT STRUCTURES 



tion to its food supply, is called the suspensor, and is a 

 temporary organ of the embryo (Figs. 141, 142, et). At 

 the end of the suspensor the real embryo develops, and 

 when its regions become organized it shows the following 

 parts : (1) a large foot buried among the nutritive cells of 

 the prothallium and absorbing nourishment ; (2) a root 

 stretching out toward the substratum ; (3) a stem extend- 



FIG. 142. Embryo of Selaginella removed from the gametophyte, showing suspensor 

 (et), root-tip (w), foot (/), cotyledons (bl), stem-tip (fit), and ligules (lig). After 

 PFEFFEB. 



ing in the other direction, and bearing just behind its tip 

 (4) a pair of opposite leaves (cotyledons) (Fig. 142). 



As the sporangia of Selaginella are eusporangiate, this 

 genus has the heterosporous-eusporangiate combination a 

 combination not mentioned heretofore, and being of special 

 interest as it is the combination which belongs to all the 

 Spermatophytes. For this and other reasons, Selaginella 

 is one of the Pteridophyte forms which has attracted 

 special attention, as possibly representing one of the an- 

 cestral forms of the Seed-plants. 



