GROWTH 



297 



in those that have been stored for a long time. It also accounts for 

 the fact that unripe seeds of many weeds germinate better and 

 more rapidly than ripe ones. The ready germination of imma- 

 ture seeds which normally pass through a period of rest, as, for 

 instance, the seeds of winter wheat, is most likely due to the fact 

 that they are in a condition of preliminary rest, which is more 

 readily disturbed than a deep rest. 



Having emerged from the state of rest, the axillary organs, the 

 roots and plumule of the embryo, begin to grow, and the reserve 

 substances deposited in the endosperm 

 and cotyledons are consumed in the 

 processes of respiration and growth, 

 which, as has been seen, are closely 

 connected. 



Since during the first stages of its 

 development the young plant is not 

 yet capable of independent nutrition, 

 the presence of such reserve substances 

 is very important for its successful 

 development. The removal of these 

 substances, for instance, by cutting off 

 the cotyledons, results in retarded 

 growth (Fig. 117) and may even cause 

 the death of a plant. Not only com- 

 plete removal but also a holding back 

 of the reserves of the seed leads to 

 slower growth. It has been shown 

 by Blackman's formula that during 

 the first stages growth proceeds accord- 

 ing to the compound interest law. 



Therefore, the dimensions of a plant are to a considerable degree 

 determined by the amount of the initial "capital," i.e., the quan- 

 tity of reserve substances and the size of the seed in general. 

 Hence seed size is of profound influence on the further develop- 

 ment of the plant. As has been often said, from bigger seeds there 

 usually develop more vigorous specimens (Fig. 118). On the 

 importance of size of seed material is based the practice common 

 in agriculture of grading and selecting the larger grains for sowing. 



89. Influence of Temperature on Growth. Thermophilous 

 Organisms. — Like all other vital processes, growth of plants is 



Fig. 117. — Seedlings of Phaseolus. 

 On the left a normal one; on 

 the right with cotyledons re- 

 moved at the beginning of 

 germination (after Osterhoul) . 



