158 BOTANICAL GAZETTE [august 



temperature was much lower, the time of fertilization ranged from 

 35 to 50 hours. This difference, no doubt, is partly due to a differ- 

 ence in growth conditions, and partly to delayed germination of 

 the pollen. During the cooler weather pollen placed on the stigma 

 at 3:00 p.m. was found dormant at 9:00 a.m. on the following day. 

 This delayed germination will cause a marked difference in the 

 time of fertilization. The pollen tube enters around the wall or 

 in the region of a synergid (fig. 15); its behavior with reference to 

 the synergid was not determined. 



Embryo. — The first divisions of the egg are transverse (fig. 

 17), and result in a filament of three cells (fig. 18). By vertical 

 walls in two planes this filament is divided into tiers of 4 cells 

 each (fig. 19). The basal tier and a part of the second tier remain 

 less active, but later form a massive suspensor (figs. 20, 21). No 

 distinct line between embryo and suspensor was made out. In 

 fig. 20 the dermatogen is being differentiated, which occurs later 

 than the octant stage. 



Endosperm. — The division of the endosperm nucleus usually 

 precedes that of the egg (fig. 15), but occasionally follows it (fig. 

 16). In fig. 17 the fertilized egg has completed its first division, 

 and 5 endosperm nuclei were counted. The endosperm masses 

 about the embryo and from this mass it extends around the wall 

 of the sac. Its later development is centripetal. Only the first 

 division of the endosperm nucleus was seen in the many ovules 

 studied. This fact indicates that the divisions in the endosperm 

 nuclei are simultaneous, but this feature was not determined. 



Sterility. — The sterility of ovules is a prominent feature 

 in Trifolium pratense. In the sterile ovules all the cells of the 

 nucellus remain vegetative and hence no embryo sacs are found. 

 All the flowers of a plant frequently develop sterile ovules only. 

 This seems to be related to moisture conditions, but more work 

 is necessary before a definite conclusion can be drawn as to its 

 cause. Plants grown in the greenhouse and well watered gave ioo 

 per cent sterile ovules. First crop heads collected from the field 

 during wet weather showed nearly ioo per cent sterility, while 

 first crop heads collected after two weeks of dry weather showed a 

 large percentage of fertile ovules. But even during dry weather 



