iio THE STRUCTURE OF ECONOMIC PLANTS 



Michx. and reports that the chalazal daughter cell undergoes one 

 more division, forming two megaspores, the inner one being func- 

 tional and the micropylar one disintegrating. In A. Cepa, accord- 

 ing to Jones and Emsweller, the chalazal daughter cell functions 

 directly as the megaspore rather than undergoing further division, 

 and a similar situation has been described by Strasburger (i8) and 

 Weber (ix) for other species of Allium. In species of Allium where 

 both bulblets and flowers occur in the inflorescences, there may be 

 some variations from this type of development. 



In the formation of the megagametophyte, the megaspore 

 undergoes successive nuclear divisions to form an eight-nucleate 

 gametophyte. The two synergids are of unequal size, the volume 

 of the larger one being several times that of the smaller, but both 

 are well supplied with food. In some species, the synergids disap- 

 pear immediately following fertilization while in others they are 

 more or less enlarged; and, according to Weber (xx), probably act 

 as nutritive cells until the embryo has developed to the two- or, in 

 some instances, a several-celled stage. In Allium Cepa, they are 

 probably nutritive for a short time. 



Embryogeny. — The development of the embryo in Allium has 

 been investigated by Soueges (17) for A. ursinum L. and by Porter 

 (13) for A. mutabile Michx. The two accounts agree in general 

 details, and the mature embryos in each case are much like the one 

 described by Hoffman for A. Cepa. The first division of the zygote 

 is transverse, producing an apical and a basal cell. (Fig. 100, A.^ 

 This is followed by another transverse division of the basal cell, so 

 that the three-celled embryo consists of an apical cell which is the 

 product of the first segmentation of the egg, and an intermediate 

 and terminal cell which result from the subsequent transverse divi- 

 sion of the basal cell. (Fig. 100, B.) Later, oblique or vertical 

 divisions of the apical cell give rise to the quadrants of the embryo 

 which by further division and differentiation produce the cotyledon 

 and growing point of the epicotyl. (Fig. 100, H, Z.) Divisions 

 of the intermediate cell produce the hypocotyledonary portions 

 of the embryo, including the root cap, and the basal cell of the 

 linear triad forms the suspensor by a series of divisions. As the 

 development of the hypocotyl proceeds, a blocking off of the his- 

 togens occurs, and in the embryo there is a clearly defined plerome 

 outside of which lies a group of meristematic cells that produce 

 the periblem, dermatogen and the root cap. (Fig. 100, /, /.) 



