CONIFERALES (TAXACEAE) 335 



or less active encroachment is striking. In regions of active invasion 

 the endosperm cells are radially elongated, and many of them are 

 binucleate; while in regions of less active invasion the cells are more 

 nearly isodiametric and rarely binucleate. 



Another proof that endosperm is the encroaching tissue may be 

 obtained from comparative measurements. At the time of fertilization 

 the gametophyte usually measures 20 by 30 /*. In the mature seed 

 the ordinary length of the gametophyte is 20 mm., the greatest width 

 being 14 mm., and the least (at the deep constriction opposite the 

 vascular strands) i . 5 mm. At this most resistant region of the 

 perisperm, therefore, where it is hard to escape the conviction that the 

 perisperm plate has cut the endosperm nearly in two, the endosperm 

 has increased its diameter against the perisperm seventy-five times. 

 The best reason, however, for concluding that the endosperm is the 

 invading tissue in this case is that this is always the behavior of endo- 

 sperm; and it is singular that the old explanation of "rumination" 

 was ever suggested. 



THE MALE GAMETOPHYTE 



The male gametophytes of podocarps and of taxads are in such 

 sharp contrast that the two tribes must be considered separately. 



Among the Taxineae the gametophyte of Taxus naturally received 

 early attention, and its principal features have been known since the 

 pioneer work of Belajeff (33) and Strasburger (38). At that time 

 it was not appreciated that the vegetative (prothallial) cells had been 

 eliminated, much less that this elimination was a feature of all the 

 taxads. 



The development of the gametophyte of Torreya taxifolia (loi) 

 will serve as an illustration and for comparison. The first division 

 of the microspore results in the generative and tube cells, separated 

 from one another by a very dehcate wall or membrane. In this 

 binucleate condition (fig. 384) the pollen grains are shed (late in March 

 to early in April in Florida). Pollen tubes penetrate the nucellar cap 

 at once, and occupy it for about four months. The generative cell 

 divides early in the history of the tube, which contains almost from 

 the first the large body cell and the stalk and tube nuclei (fig. 385). 

 Shortly before fertilization (about the middle of August), the body 



