38o THE STRUCTURE OF ECONOMIC PLANTS 



Because of the variation in the divisions of the two median cells 

 of the linear tetrad and the consequent variation in the orientation 

 of the derivative cells, three distinct types of eight-celled pro- 

 embryos occur. The most common one is an octant which has 

 seven linear layers of cells; but other possibilities include ones 

 with six or with five layers. (Fig. 193, 13, 15.) All later stages 

 in the embryonic development can be referred to one of these three 

 tvpes, the derivatives of the more common seven-layered type are 



Fig. 19}. Early stages in development of embryo of Linum catharticum L. : ac and be, 

 apical and basal cells of the proembryo ; m and ci, cells derived from basal cell ; / and /', cells 

 derived from apical cell ; « and «', cells derived from ci; d and /, cells derived from m: cal- 

 dgn, calyptrogen-dermatogen ; h, hypophysis; ^hm, periblem ; p/, plerome; sus, suspensor. 

 (Redrawn and adapted from Soueges, Compt. Rend. Acad. Sci.^ 



shown in Figure 193, 19, lo, ii, and those of the six-layered type 

 in Figure 193, 18, 2.1. 



In the subsequent development of the embryo, the cell /, or the 

 upper daughter cell derived from it, undergoes periclinal division, 

 and further development of the resultant cells leads to the differ- 

 entiation of the cotyledonary portion of the embryo. The behavior 

 of the cells /' and m of the original tetrad varies with the type of 

 eight-celled proembryo; but in the most common case (the seven- 

 layered proembryo) the cell /' divides into two superposed cells, 

 the upper one producing the hypocotyl, while the lower one 

 divides again transversely, forming two cells. The upper of these 



