3" 



BOTANY OF THE LIVING PLANT 



surface (anticlinal). Their position is uniform, and the first is usually 

 longitudinal (ii.), but their succession may vary. This suggests that 

 no grc.it morphological value can be set on their order of appearance 

 (iii. iv.). Later each octant divides into an outer and inner cell, by a wall 

 parallel to the surface (periclinal) ; the superficial cells thereafter divide 

 only by anticlinal walls, and the layer thus produced is called dermato- 



gen, because it forms the epidermis 

 (v.-viii.). The inner cells divide 

 again periclinally to form an inner 

 and an outer series ; this is more 

 regular in the lower tier of octants, 

 which will form the hypocotyl and 

 root. The inner series constitutes 

 the plerome, which forms the 

 stele ; the outer is the periblem, 

 which forms the cortex (vi.-viii.). 

 Meanwhile the cell of the pro- 

 embryo adjoining the lower tier 

 of octants (the hypophysis, here 

 shaded) has enlarged, and divided 

 (vi. vii.), so as to form a group of 

 cells which encroach into the 

 spherical embryo. It provides 

 the apex of the root, which is thus 

 attached to the suspensor, and it 

 is always directed towards the 

 micropyle. The upper tier of 

 octants soon gives rise to two pro- 

 jecting lobes (cotyledons), which 

 bear no constant orientation 

 relative to the first segmenta- 

 tions. Between them is a smooth 

 groove, where the plumule will arise later. It is now possible to 

 recognise the position of all the parts of the germ, viz. the radicle, the 

 two cotyledons, and the plumule between them. In the Shepherd's 

 Purse the seed is exalbuminous (p. 317), and the embryo develops fast 

 in bulk, and in length (Fig. 231). But the ovule is of the type with a 

 :urved embryo-sac. The embryo, as it grows, adapts itself by curving 

 Iso, and soon fills the greater part of the sac. Meanwhile the plumule 

 at last appears at the base of the groove between the cotyledons, 

 position coincides with the intersection of the octant walls. 



Fig. 231. 



Shepherd's Purse. Photomicrograph of young 

 seed, showing embryo, endosperm centrally, and 

 developing testa on the outside. ( x 125.) The 

 micropyle is directed upwards and to the left, 

 and the root-tip is directed towards it. (After 

 Coulter and Chamberlain.) 



