174 THE STRUCTURE OF ECONOMIC PLANTS 



the second microgamete in all cases in Triticum vulgare, Sax (13) 

 observed the phenomenon in Triticum durum. It is possible that 

 the fusion of the polar nuclei with the second microgamete may 

 take place so rapidly that this stage is difficult to observe. The 

 antipodal cells show signs of disintegration at the time of fertiliza- 

 tion; and, as the embryo and endosperm develop, the surrounding 

 nucellar tissue is also gradually resorbed. The outermost layer of 

 the nucellus persists as an actively dividing tissue for some time, 

 but ultimately its cells become disorganized and the radial walls 

 are broken down. In the mature grain, it may appear as a crushed 

 layer outside the aleurone cells of the endosperm. 



Embryogeny. — In the development of the embryo, the first 

 division of the zygote is transverse, and the large, basal, daughter 

 cell functions as the suspensor which does not divide again. The 

 upper cell forms the embryo, dividing first in the transverse plane, 

 and then a vertical wall is formed in both the cells so that the pro- 

 embryo consists of four cells with a basal suspensor. Further 

 divisions in all planes result in a marked increase in the size of the 

 embryo; and, in a few days, the dermatogen can be distinguished 

 surrounding a central body of cells. 



At this time, the embryo is club-shaped with an elongated base, 

 and the first form change occurs in the development of a lateral 

 notch indicating the position of the growing point of the axis 

 which appears on the side of the embryo away from the endosperm. 

 (Fig. 79, B.) The distal portion of the embryo becomes the coty- 

 ledon or scutellum, which Percival regards as a terminal structure. 

 A more generally accepted interpretation is that the growing point 

 is terminal and the cotyledon a lateral structure; which, because 

 of the slow growth of the apex and the rapid growth of the coty- 

 ledon, appears to occupy a terminal position. The position of the 

 suspensor at the base of the axis, rather than at the lower portion 

 of the cotyledon, supports the latter view. 



Shortly after the apical meristem of the stem is defined, the pri- 

 mordium of the coleoptile arises as a fold of tissue surrounding the 

 stem tip and forms a cone-shaped structure which encloses the 

 primordia of the first foliage leaves. (Fig. 79, C, D.) At this 

 time, the epiblast arises as a lateral outgrowth in the region of the 

 cotyledonary node. There is intercalary growth at the base of the 

 upper portion of the scutellum which elongates the apex, resulting 

 in the formation of the overhanging ventral scale; and the develop- 



