THE ORIGIN OF THE SEED HABIT 391 



The retention of the megaspore in the megasporangium was, 

 perhaps, the most important step in the development of the seed 

 habit. This retention was possibly at first somewhat accidental; 

 that is, the megaspore simply failed to fall out of the megaspo- 

 rangium (as actually happens in some species of Selaginella, 

 Sec. 325), and consequently developed its gametophytes while 

 mechanically held on the sporophyte. Later the retention be- 

 came more intimate and less mechanical, so that the female 

 gametophyte established a close physiological association with 

 the sporophyte, obtaining protection and certain foods, and per- 

 haps most important of all it was kept moist. At last the mega- 

 spore, instead of being developed as a free cell, remained a part 

 of the tissue of the megasporangium (nucellus of the ovule) and 

 at that stage became the embryo sac with its clearly estab- 

 lished parasitic relations to the sporophyte. 



Some forms of Selaginella actually illustrate a beginning of 

 such parasitic relations in the early stages of the development 

 of its megaspore (Sec. 325), for the female gametophyte begins 

 to develop before the spores are full grown and ready to be 

 discharged. But the seed habit could not have been entirely 

 formed until the megaspore became physiologically a part of 

 the megasporangium, and the latter (as a nucellus), together 

 with the protective integuments, became the ovule. 



The development of the pollen tube is perhaps even more 

 remarkable than the retention of the megaspore in the mega- 

 sporangium. It seems clear that the pollen tube is a develop- 

 ment in response to the stimulus of the moisture (containing 

 food substances) which is excreted by the ovule in the gymno- 

 sperms and the stigma of the angiosperms. The habit may 

 readily have had a very simple beginning if microspores fell 

 into partially opened megasporangia, as indeed occurs in a spe- 

 cies of Selaginella (Sec. 325), or among a group of megasporo- 

 phylls. They would have found in such situations moisture 

 and other conditions favorable for the development of out- 

 growths which later became tubes. These outgrowths and 



