398 



FO UR TH GR UP.— SEED-PLANTS. 



transverse wall into two cells, of which the one next the suspensor takes no further 

 part in the development of the embryo. The upper of the two cells (the hypophysis 

 of Hanstein) divides into two cells lying one above the other by a transverse wall 

 which is curved Hke a watch-glass (Fig. 327). These two cells are first divided by 

 longitudinal walls, as is shown in Fig. 326 V, and then the lower one of them by a 

 transverse wall. The cells which proceeded from the upper of the two cells of the 

 hypophysis form the termination of the periblem of the root, while the lower one 

 gives rise to a layer of cells, shaded in Fig. 326 F, which connects with the derma- 

 togen and forms the first layer of the root-cap ; as Fig. 329 shows, the root-cap is 

 produced by the continued growth of the root accompanied by corresponding 

 divisions by cell-walls, and may therefore be described simply as a luxuriant growth 

 of the dermatogen. The peripheral layer of tissue, which elsewhere remains single 

 and passing into permanent tissue forms the epidermis, becomes thicker where it 

 covers the growing point of the root, and suffers tangential divisions (parallel to the 

 surface) which are repeated periodically; of the two layers thus formed at each 



Fig. 327. Capsella Bursa 

 fastoris. Embryo about half de 

 veloped (shortly before the ap 

 pearance of the cotyledons) in op 

 tical longitudinal section, some 

 what diagramraatically rendered. 

 The hypophysis has divided intc 

 the cells A and A]. After Hanstein 



Fig. 328. Embryos of Ovobanchc. The cell-divisions agree on the 

 whole with those in Capsella, but among other variations the anticlinal line 

 first makes its appearance in the left upper octant, and then the periclinal P. 

 Similar irregularities occur also sometimes in Capsella. h hypophysis, P the 

 periclinal wall which divides off the dermatogen. After Koch. 



division the outer becomes a layer of the root-cap (Fig. 329, 2 and Fig. 333 wh),, 

 while the inner remains dermatogen and repeats the same process. The dermatogen 

 which covers the vegetative cone of the root behaves therefore in the same way as a 

 layer of phellogen, but with this difference, that the cells produced by the cork- 

 cambium become at once permanent cells, whereas those of the root-cap continue 

 capable of division, and thus each layer of cells separated off from the dermatogen 

 gives rise to several layers of cells of the root-cap, the growth of which is most active 

 in the centre and lessens towards the circumference. The formation of the root-cap 

 is different from this in other Angiosperms, and will be touched upon presently. 



Alisma Plantago may serve as an example of the development of the embryo 

 in a monocotyledonous plant. Fig. 330 / shows the stage in which the embryo 

 consists of three cells, the proximal cell {q) of the pro-embryo being in this case swollen 

 out into a vesicle. The middle cell (r) now divides by a number of transverse walls 



