56 SEEDS AND FRUITS 
young plant, which after reaching a certain stage of development, 
varying in different plants, passes into a dormant stage from which 
it may awake if conditions are favorable and continue its devel- 
opment until it becomes a mature plant. In the development of 
the embryo from the fertilization of the egg to the dormant stage, 
certain structures which function in the further development of 
the young plant are usually more or less developed. In a well 
formed embryo like that of the Bean, there are four parts, hyocotyl, 
plumule, cotyledons, and radicle. In Figure 60 of the Bean, h is 
hypocotyl, p, plumule, and c, cotyledons. The radicle (r) is at 
the lower end of the hypocotyl and is so closely joined with the 
hypocotyl that it does not appear as 
a separate structure. The cotyledons 
of the Bean have absorbed the endo- 
sperm and consequently are so much 
enlarged that they form the bulk of 
the embryo. The special functions 
performed by the different parts of the 
embryo are quite noticeable in the 
germination of the seed. The cotyle- 
dons supply food; the plumule develops 
Fic. 60.— Bean with testa stem and leaves; the radicle develops 
removed and cotyledons 4 roo4- and the hypocotyl in many 
spread apart. c, cotyledons; 
h, hypocotyl; p, plumule; r, C488 pulls the cotyledons and plumule 
radicle. out of the seed coat and raises them 
above ground. 
The stored food and seed coat are temporary structures. They 
nourish and protect the young plant in its early stage of develop- 
ment and then disappear. The stored food, consisting chiefly of 
starch, proteins, and oils, the proportion varying in different 
seeds, develops in close contact with the embryo and when not 
absorbed as rapidly as it develops, it forms the storage tissue or 
endosperm in which the embryo becomes imbedded. The testa, 
the protective structure of the seed and usually formed from 
the integuments of the ovule, generally consists of a single 
covering so much thickened and hardened that it protects the 
embryo against injuries. Often there is a thin inner covering 
and in exceptional seeds, like those of the Water Lily, an extra 
outer covering called the aril develops later than the integuments 
and forms a loose covering about the seed. (F7g. 62.) 
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