GROWTH OF PLANTS 



113 



r€L>' 



growth are localized in different regions of most seeds. From a 

 physiological point of view, there may be distinguished three 

 main parts of the seeds: (1) the seed coats covering the seed 

 from the outside and serving as a protection from various injuries 

 and unfavorable external conditions; (2) the embryo, consisting 

 of a rudimentary rootlet, leaves, and a stem uniting them; and 

 (3) reserve tissues, occupying usually 

 the greatest part of the volume of the 

 exalbuminous seed, when the reserves 

 are stored mainly outside of the 

 embryo itself in the endosperm. The 

 endosperm may be vanishingly small 

 in seeds whose reserves are stored in 

 the cotyledons (albuminous seeds). 



Storage in the cotyledons can be 

 observed in seeds of peas, beans, and 

 in most dicotyledonous plants (Fig. 

 21). Storage in the endosperm can 

 be observed in seeds of cereals and 

 grasses. The endosperm is of an 

 origin quite different from that of the 

 embryo. It originates not from the 

 fertilized egg but from the union of 

 primary endosperm nuclei, and sub- 

 sequent cell divisions after fusion wdth 

 the second generative nucleus of the 

 pollen tube. The endosperm nucleus . ^^^- ^^-^^^^^ ?^ beans: 



. . A— externally; B— from the 



IS thus tormed by the fusion Ot three internal side of the cotyledon; 



nuclei and has 3n number of chromo- (>-growth of the embryo parts 



of the seed. 



somes. Ihe endosperm represents a 



uniform parenchymatous tissue almost entirely filling the seed of 

 cereals, the embryo being pushed to one end. This structure 

 may be observed in the seeds of most monocots (Fig. 22). 

 Besides these tw^o main types of seed, there exist also others of 

 less importance. 



The germination of seeds begins with swelling. With the 

 development of the embryo, the seed coat bursts, ceasing to 

 impede further growth. The growth of embryonic parts of 

 seeds proceeds at the expense of the reserve substances, which 

 are decomposed by enzymes in the processes of hydrolysis. The 



