434 THE HOME, FARM AND BUSINESS CYCLOPAEDIA. 



night, while the roots are still at work, the transpiring power of the leaf 

 is lessened, and drops of water exude from the leaves. Where the tem- 

 perature is so high as to kill the plant or leaf outright, it is the proto- 

 plasm which dies ; its constitution and molecular construction become 

 changed, its power of absorbing water destroyed, and thus the turgid 

 condition of the cells is lost. 



INFLUENCE OF HEAT AND MOISTURE ON THE STEM. The growth of the 

 stem is directly influenced by heat, there being in this as in other cases 

 a minimum below which growth cannot take place, an optimum at which 

 it takes place most vigorously, and a maximum beyond which heat is in- 

 jurious. The favourable influence of heat it is which in part overcomes 

 the influence of gravitation, and enables the stem to ascend. The stem 

 will grow fastest and strongest on the side most exposed to the heat, if 

 that heat be net excessive, and this tendency will remove it from the soil. 

 Similarly a moist condition of the atmosphere favours growth, and the 

 stem will grow the faster on the side most exposed to the moist vapour, 

 and, owing to the convexity so formed, it will in consequence bend its 

 free end and its concavity towards the drier side. 



GERMINATION. The conditions under which germination takes place 

 need not be alluded to at any length, as they are the same as those re- 

 quisite for growth, and practically every cultivator knows that air (oxy- 

 gen), moisture, and heat, varying in amount according to the plant and 

 according to circumstances, are required, and that his success depends in 

 great measure upon the proper tillage of the soil which secures these 

 requisites. When the seed, or rather the* embryo plant within it, begins 

 to grow, water is absorbed, the seed swells, the insoluble starch stored up 

 becomes converted into glucose, or a form of sugar, by the agency of a 

 nitrogenous substance which acts as a ferment. These chemical processes 

 are accompanied by an evolution of heat and an outpouring of carbonic 

 acid gas. Thus is it that in malting barley the grain swells, gets hot, and 

 its starch is converted into sugar. As the seedling grows, both starch 

 and sugar gradually disappear, although the stock of starch is continually 

 replenished so long as the leaves continue to act. The nitrogeneous con- 

 stituents of the seed undergo similar changes from the insoluble to the 

 soluble condition, the latter being capable of transport from place to place 

 as may be required. 



FERTILISATION. In the case of plants grown for their fruit or seed, as 

 in the case of wheat and cereals generally, much attention has naturally 

 to be paid to the conditions which favour sexual multiplication. 



