SWEET-CLOVER SEED. 7 



2, and 3.) In general, the development of the embryo sac proceeds 

 in the ordinary way^ as described by Young (44, p. 133), with the 

 inner megaspore functioning. (Text fig. 4 and PI. II, fig. 1.) In its 

 development the nucellus is destroyed rapidly, the destruction being 

 most rapid first at the micropylar end proceeding backward. The 

 nucellus is completely destroyed at the micropylar end by the time 

 the embryo sac is mature, and consequently the embryo sac comes in 

 contact with the outer integument in this region. (PI. II, fig. 1.) 

 As the destruction of the nucellus extends toward the chalazal end 

 the embryo sac becomes much elongated and tubelike. The antip- 

 odals disappear early, so that a mature embryo sac consists of the 

 egg, the synergids, and the two polars. The two polars lie in contact 

 in the micropylar end of 

 the sac near the egg until 

 fertilization. 



STERILITY OF THE OVULES. 



In Melilotus alba and M. 

 officinalis there is very 

 little tendency toward 

 sterility of ovules. In an 

 extended study of ovules 

 developing under normal 

 and under excessive mois- 

 ture conditions only an 

 occasional one was found 

 in which no reproductive 

 cells were differentiated, 



anCi no Ovaries Were IOUnd yig. 4.— Median section through an ovule, showing the embryo 

 ill which all Of the OVules sac w * tn f° ur nuc lei and the position of the integuments. 

 ., X150. 



were sterile. 



DEVELOPMENT OF THE POLLEN. 



The pollen mother cells do not separate, but previous to the reduc- 

 tion division the protoplasm shrinks from the walls, thus forming a 

 dense globular mass which often occupies less than half the lumen of 

 the mother cell. (PI. I, fig. 4.) Nuclear division occurs while they 

 are in this contracted condition, and four nuclei are formed from two 

 successive divisions. The cytoplasm is equally distributed around 

 each nucleus. The four masses of protoplasm separate, and as they 

 enlarge a number of times and develop into mature pollen grains they 

 become binucleate, and a wall is gradually formed around each. 

 (PL I, figs. 5 and 6.) At first the cytoplasm is quite dense and con- 

 tains some starch but no fatty oils. However, the cytoplasm of 



