1914] MARTIN— LEGUMINOSAE 159 



there is considerable variation among plants, some producing 

 nearly all fertile ovules, and others a large percentage of sterile 

 ones. There is a marked tendency toward sterility, which seem 

 to be favored by moisture. This tendency, no doubt, always 

 lowers the percentage of seed production, and in some cases 

 reduces it almost to zero. The fact that this tendency varies 

 among plants under similar conditions suggests that it may be 

 partly eliminated by selection. A sterile ovule is shown in fig. 22. 

 The flower was open and the embryo sac should have been ready 

 for fertilization. Sterile ovules can be identified only in later 

 stages. The cells of the subepidermal rows are usually larger 

 and less dense in content, but it is safe to pass judgment on the 

 earlier stages only when all the later stages of the head are sterile. 

 No mother cells in synapsis were found in the sterile ovules, so 

 sterility seems to be determined before this stage is reached. 



Parthenogenesis.— Trifolium pratense has been reported 

 parthenogenetic. Flowers were run under cover and killed at 

 various times after wilting. An examination of more than 500 

 ovaries showed no embryos. The ovule enlarges very rapidly for 

 several days after the embryo sac is ready for fertilization and then 

 begins to break down. 



Trifolium hybridum 



This species is so similar to Trifolium pratense that a few 

 features only deserve mention. The number of ovules in an ovary 

 is variable, ranging from 3 to 8. Fig. 23 shows the lower mega- 

 spore germinating before the others are destroyed. The embryo 

 sac has a large central vacuole and the cytoplasm is almost entirely 

 limited to a thin peripheral layer (fig. 24). The embryo sac be- 

 comes more curved than that of Trifolium pratense (fig. 27). The 

 polars fuse in a parietal position (fig. 24). The proembryo is 

 more slender and there is a more definite line between embryo 

 and suspensor in the later stages (fig. 25). Fig. 25 also shows the 

 taint walls that sometimes occur in the early development of the 

 endosperm. Fig. 26 shows the suspensor on the hypocotyl of the 

 embryo. The tendency toward sterility is not so pronounced in this 

 species as it is in Trifolium pratense. 



