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The Embryology of Melilotus Alba. 



By W. J. Young. 



In the space assigned to this paper it will be possible to give but a 

 brief outline of the development and nutrition of the flower, embryo sac 

 and embryo, trusting to the plates to make clear the points not sufficiently 

 explained in the text. 



In the flower of Melilotus alba, the common white sweet clover, 

 there are two factors which interfere with the orderly successive develop- 

 ment of the floral organs; the one is mechanical and is due to the crowd- 

 ing of the flowers on the side of a stem axis, the other is physiological 

 and ecological and depends upon the relative use of parts at different 

 times in the development of the flower; the one interferes with the 

 simultaneous appearance of the parts of the same cycle, the other with 

 the acropetal succession. 



Melilotus forms an exception to most of Leguminosae so far ex- 

 amined, since the megaspore mother cell develops into the embryo sac 

 without first undergoing tetrad division. The development of the latter 

 differs from typical cases only in details. When the eight-celled stage is 

 reached, the embryo sac elongates rapidly toward the chalaza at the same 

 time curving strongly, and the three antipodal cells disappear. The egg 

 cell occupies a position lateral to the synergids. The polar nuclei lie 

 close together near the egg apparatus. Their fusion takes place just 

 prior to fertilization, before the pollen tube reaches the ovule. 



After fertilization the primaiy endosperm nucleus undergoes several 

 divisions before the egg cell begins to divide. The latter then undergoes 

 two transverse divisions, resulting in a terminal cell which develops into 

 the embryo and two other cells which give rise to a conspicuous suspensor. 

 The embryo follows in the main the Capsella type of development, but 

 with two important differences, viz., much later differentiation of the 

 dermatogen, plerome, and periblem, and the absence of a hypophysis 

 derived from the terminal cell of the suspensor. 



Lack of space prevents more than the merest glance at the facts 

 observed regarding the nutrition of the embryo sac and embryo. In its 



