FAMILIAR SEEDS, STRUCTURE AND GERMINATION 3 
be fulfilled. The seed must be supplied with water, the 
temperature must be between certain limits, in the case 
of the bean seed between 5° C., and 88° C., and it must 
have free access to oxygen. Growing plants breathe 
just as animals do, and hence oxygen is as necessary 
for their life and growth as it is in the case of animals. 
The first thing that can be noticed when a bean seed 
germinates is the bursting of the testa by the radicle near 
the micropyle and the elongation of this part downward. 
After the radicle has reached a certain length, the 
plumule forces its way out from between the cotyledons, 
pushing aside a flap of the seed coat, but keeping its 
upper end curved (fig. 1 c) in order to protect the 
delicate tip of the plumule. After some time the 
plumule straightens itself and the leaves surrounding 
the tip of the plumule begin to unfold. The cotyledons, 
remain below the ground, giving up their reserve food 
material to the rapidly growing seedling, and conse- 
quently becoming smaller and smaller. All this time 
the radicle has been increasing in length and giving off 
lateral branches. The main root, i.e. the root produced 
by elongation of the radicle, is called the Tap Root. It 
not only bears lateral roots, but also gives rise to a 
number of fine hairs called Root-Harrs just behind 
the tip. 
3. The Castor-Oil Seed.—Although the external ap- 
pearance of the castor-oil seed does not differ much from 
that of the broad bean, there are several differences in 
the internal structure (fig. 2). The testa itself has a 
hard outgrowth at one end called the Ariz. If we 
cut a longitudinal section of the seed we find that the 
1 * 
