280 A TEXTBOOK OF GENERAL BOTANY 
Endosperm. In some seeds, such as the squash seed and the 
lima bean, the cotyledons are large and fleshy and are filled with 
stored food (Figs. 268, 270). In others the cotyledons are thin 

Fig. 268. Lon- 
gitudinal — sec- 
tion of squash 
seed 
c, cotyledon ; p, 
plumule; 7, rad- 
icle; tm, tegu- 
ment ; ts, testa ; 
m, micropyle. 
(x 8) 
of fertilization. 
and are surrounded by a mass of food-storage 
tissue, the endosperm, as in the seed of the castor- 
oil plant shown in Fig. 269. Most seeds of flower- 
ing plants contain an endosperm at some stage 
of their development, and the food stored in this 
endosperm is absorbed by the cotyledons. In 
many cases the endo- 
sperm is completely 
absorbed before the 
seeds are mature, and 
in such cases the ripe 
seed does not contain 
endosperm (Figs. 268, 
270). In other cases 
much of the food 
material in the endo- 
sperm is not absorbed 
by the cotyledons un- 
til the seeds germi- 
nate. In these cases 
the mature seed con- 
tains an endosperm. 
ae _ Above: ca, caruncle ; f, hilum. 
The endosperm has Below: at left, section cut from 
its origin at the time _ sidetoside; at right, section cut 
After the p ollen tube perpendicular to dorsal surface ; 
r, radicle ; p, plumule; c, coty- 

Fig. 269. Ventral, dorsal, and 
lateral views of seed of castor- 
oil plant (Ricinus communis), 
with section of seed _ 
has grown down into the ovule it jean; ¢, endosperm ; tm, tegu- 
discharges two male nuclei into the ment; ts, testa. (x 2) 
embryo sac. One of these unites with 
the nucleus of the egg, and the fertilized egg develops and 
becomes the embryo. The other male nucleus unites with the 
two polar nuclei. This fusion nucleus undergoes divisions result- 
ing in the production of the endosperm tissue in the embryo sac 
around the embryo. 
