DORMANCY OF SEEDS 



841 



endosperm is imperfectly developed, e.g., the pea, food 

 storage occurs in the seed leaves or cotyledons of the 

 embryo plant. The tissues of the ovule continue growth 

 along with the young plant and become differentiated to 

 form a. hard coat which serves to protect the embryo. - 



P«» seed, with sead coat r«- 



and ootyledons sepa- 

 ratad to show jQunii »t«3i 

 and root. 



Fig. 94. — The Structure of Seeds. The diagrams show three 

 types of seed. (1) Pine seed in which the embryo is located in^the 

 middle of the large female gametophyte and has most of the food 

 stored in the gametophyte and some in the embryo; (2) the com 

 grain which has food stored largely in the cells of the endosperm; 

 , and (3) the pea seed, in which there is nothing left either of. the 

 female gametophyte or of the endosperm, has the food stored in 

 the much enlarged seed leaves or cotyledons. Note also that the 

 embryo of the pine has several cotyledons of which the diagram 

 shows two, one on either side of the stem tip; that the corn embryo 

 has bift one very large cotyledon or seed leaf and that the stem tip 

 occupies an apparently lateral position; and finally that the pea 

 has two large lateral cotyledons and a terminal stem tip. 



Dormancy of Seeds. — The seed differs from most 

 parts of the plant in that it can become air dry without 

 injury to the protoplasm of its cells and then resume 

 growth when placed in a suitable temperature and pro- 

 vided with sufficient water. This is a very important 

 circumstance for it enables a plant to survive long-con- 

 tinued unfavorable conditions for growth or to be carried 

 long distances without injury. Seed Plants have thus 

 a great advantage over Ferns, Mossed, and the lower 



