236 EMBRYOGENESIS IN PLANTS 



Johansen (1950) recognises a sixth type — the Piperad type — in which 

 the first wall is longitudinal or nearly so. He also substitutes the term 

 Onagrad type for Cnicifer type. 



These several types are indicated diagrammatically in Fig. 55. 

 Various embryos from different orders are illustrated in Figs. 56-59. 



Thanks to the investigations of Hanstein (1870) and Famintzin 

 (1879) and later of Soueges (1914, 1919), the embryology of CapseUa 

 bwsa-pastoris, though by no means generally representative, has 

 become a classical example of dicotyledonous embryonic development. 

 It also happens to be a useful and convenient 'teaching type' and as 

 such may be briefly described here. The first division of the zygote is 

 by a transverse wall. The terminal or distal cell now divides longi- 

 tudinally and the basal cell transversely, so that the proembryo consists 

 of four cells. The two terminal cells now divide by a longitudinal wall at 

 right-angles to the first, a quadrant stage being thus constituted. 

 Division of these cells by transverse walls yields the octant stage, the 

 distal region of the embryo now consisting of a small growing sphere. 

 Fig. 53c. The distal four octant cells on further development give rise 

 to the shoot apex and two cotyledons; and the proximal four cells to 

 the hypocotyl. All the eight cells enlarge considerably and divide by 

 periclinal walls, the outer cells forming the dermatogen, the inner ones, 

 after further divisions, giving rise to the periblem and plerome initials, 

 Fig. 53d. Contemporaneously, the two basal cells elongate and under- 

 go a number of transverse divisions, an elongated suspensor of 6-10 

 cells being formed. The proximal cell, i.e. the cell which is situated 

 close to the micropyle, becomes conspicuously enlarged and is usually 

 considered to have a haustorial function. The most distal of the 

 suspensor cells, which is contiguous with the spherical distal region, 

 continues to develop and functions as the hypophysis. This cell enlarges, 

 projects a little into the spherical distal region, divides by a transverse 

 wall and the two daughter cells by two vertical walls at right-angles to 

 one another, a group of eight cells being thus formed. Fig, 53d, e, f. 

 The most distal group of four cells forms initials of the root cortex, 

 while the proximal group gives rise to the root-cap and the root 

 epidermis, Fig. 54. The spherical embryo continues to enlarge with 

 further divisions of its constituent cells. As the cotyledons begin to 

 develop in the distal region, the spherical embryo becomes transformed 

 into a somewhat flattened cordate body. Both the cotyledons and the 

 hypocotyl now begin to elongate, chiefly by transverse divisions of their 

 cells; the nascent shoot apex consists of a small-celled region, situated 

 in the depression between the cotyledons. The root apex is meanwhile 

 becoming organised, and incipient vascular tissue can be seen in the 

 hypocotyl between the shoot and root apices. The enlarging embryo 



