276 



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 great 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 is called dermatogen, 



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 are the 

 plerome, which forms the stele ; 

 the outer are the periblem, which 

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

 while 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 em- 

 bryo. 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 projecting lobes (cotyledons), 

 which bear no constant orienta- 

 tion relative to the first segmen- 

 tations.. Between them is a smooth 

 groove, where the plumule w 

 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. 281), and the embryo develops faot in bulk, and in length (Fig. 218). 

 But the ovule is of the type with a curved embryo-sac. The embryo, 

 as it grows, adapts itself by curving also, and soon fills the greater part 

 of the sac. Meanwhile the plumule at last appears at the base of 

 the groove between the cotyledons. Its position coincides with 

 the intersection of the octant walls. Accordingly its position was 



FIG. 218. 



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. 

 (After Coulter and Chamberlain.) 



