io8 THE STRUCTURE OF ECONOMIC PLANTS 



Schaffner (9) for Sagittaria; and Soueges (10) for Allium, outline 

 the general sequence of events for dicotyledons and monocoty- 

 ledons respectively; but the details should be described for each 

 plant individually. 



In the dicotyledonous embryogeny, the zygote usually divides 

 transversely to form an apical and basal cell; and subsequent trans- 

 verse divisions, which may involve the apical cell or both daughter 

 cells, result in the formation of a proembryo consisting of a filament 

 of cells of varying length. In most cases, the derivatives of the 

 apical cell of the proembryo form the embryo; while the basal cell 

 or its derivatives and the remaining linear cells of the proembryo 

 form the suspensor. The first division of the apical cell of the 

 proembryo is usually longitudinal. This is followed by a second 

 division in the longitudinal plane, at right angles to the first divi- 

 sion, and a transverse division, or the order may be reversed. In 

 either case, the octant stage is reached and the transverse division 

 delimits the cotyledonary from the hypocotyledonary portion of 

 the embryo. 



Following the octant stage, the histogens may be differentiated, 

 and the dermatogen is cut off by periclinal divisions which occur 

 first in the terminal portion of the embryo and progress toward the 

 hypocotyledonary region. The differentiation is continued rapidly 

 so that the dermatogen is soon complete except at the apex of the 

 primary root which is in contact with the adjacent cell of the sus- 

 pensor. This portion of the dermatogen is usually derived from the 

 uppermost cell of the suspensor, and the periblem and plerome then 

 differentiate within the dermatogen. The epicotyl and cotyledons 

 are derived from the four apical octants of the young embryo, while 

 the major portion of the hypocotyl is derived from the four basal 

 ones. The embryo rapidly develops its hypocotyledonary axis and 

 cotyledons, the degree of differentiation which it may attain 

 being determined largely on the basis of the speed with which the 

 fruit and seed ripen. 



In a monocotyledonous embryogeny, as represented by Allium, 

 the first division of the zygote is transverse, producing an apical and 

 basal cell. This is followed by a second transverse division of the 

 basal cell so that a three-celled embryo is formed. The subsequent 

 divisions of the apical cell do not appear to be entirely regular; 

 but, as a result of oblique or vertical divisions of the apical cell, 

 quadrants of the embryo are formed which produce the cotyledon 



