THE WALLFLOWER 



139 



with the seeds attached to it, and the papery septum 

 stretched across it (Fig. 49*, 3). 



The lines along which dehiscence is to take place 

 are marked out beforehand as thinner bands in the 

 wall (see transverse section, Fig. 50, d, d). A special 

 layer of separation is formed at these places, the cells 

 of which split apart during dehiscence, and so release 

 the valves. 



The seeds hang on to the /' c 



replum for a time and then 

 fall, or are blown off by the 

 wind, on to the ground. 



FIG. 51. Germinating seed 

 of Wallflower, t, t, burst 

 testa ; c, c, cotyledons ; h, 

 hypocotyl ; r, root. Mag- 

 nified about 5. (R. S.) 



k. Germination 



When the temperature is 

 sufficiently high, and the 

 weather is moist enough for 

 the seeds to take up a supply 

 of water, the process of germi- 

 nation begins. By germination 

 is meant the development of 

 the seedling from the embryo contained in the seed. 



The mucilaginous layer of the testa takes up water 

 in large quantities, swells into a slimy coating, and 

 thus helps to fix the seed to the soil. Part of the 

 water which is thus taken up is passed on to the 

 embryo inside, and stimulates it after its long rest to 

 renewed growth. The hypocotyl and radicle begin to 

 elongate rapidly, and the apex of the latter is forced 

 out through the micropyle. In whatever position the 

 germinating seed may lie, the radicle as soon as it 



