6 



BULLETIN 844, IT. S. DEPARTMENT OF AGRICULTURE. 



just being differentiated and while the outer integuments are barely 

 prominent at the base of the nucellus. The pollen grains are formed 

 while the embryo sac is beginning to develop. The division of the 

 megaspore mother cell does not occur until a number of days later, 

 and the embryo sac is not mature until the flower is nearly ready to 

 open. Thus, the pollen grains are formed a week to 10 days before 

 the embryo sac is ready for fertilization. The pollen grains increase 

 in size and undergo internal changes after their formation. These 

 changes, which are not completed until the flower is one-half or more 

 of its mature length, may be regarded as the ripening processes, and 

 they are undoubtedly necessary before the pollen is capable of func- 

 tioning. For this reason it is probable that the pollen grains are not 

 able to function much before the embryo sac is mature. 



The pistils of Meli- 

 lotus alba and M. offic- 

 inalis are straight for 

 the greater part of 

 their length, but curve 

 rather abruptly toward 

 the keel just below the 

 capitate stigma. The 

 surface of the stigma 

 is papillate. (Fig. 3.) 

 In their reaction with 

 Sudan III, alkanin, and 

 safranin the walls of 

 the pa pilke of the stig- 

 ma show that some fatlike substances are present. Aside from water, 

 the contents of the papilke consist chiefly of a fine emulsion of oil. 



DEVELOPMENT OF THE OVULES. 



The number of ovules in the ovary of Melilotus alba varies from 

 two to five; however, most commonly, three or four ovules occur. 

 In Melilotus officinalis the number in each ovary ranges from three 

 to six. In both species the ovules are campylotropous at maturity 

 with the micropylar end turned toward the base of the ovary. 



Mature ovules contain two integuments, but the inner one does 

 not close entirely around the end of the nucellus. The outer integ- 

 ument develops considerably ahead of the inner one. The outer 

 integument is much thickened at the micropylar end, the seed coat 

 is formed from it, and the inner integument is used as nourishment 

 by the endosperm and embryo. 



The number of megaspore mother cells in an ovule varies from 

 one to many. Two or more embryo sacs often start to develop in 

 the same ovule, but seldom more than one matures. (PI. I, figs. 1, 



Fig. 3. — Stigma at the time of pollination, showing its papillate 

 character and the position of the pollen in reference to the papilla; 

 in pollination. X175. 



