UNEQUAL PRODUCTION OF CONSTITUENTS. 18t 



for, by the increased size attained by the plant in a given time, 

 there is also given, in fact, space for the reception of the consti- 

 tuents of the soil necessary for the formation of new leaves and 

 twigs. 



From the surface of young plants a constant evaporation of 

 water takes place, the amount of which is in proportion to the 

 temperature and surface. The numerous fibres of the roots 

 supply the water which is evaporated, just as if they were so 

 many pumps ; so that, as long as the soil continues moist, the 

 plants receive, by means of water, the necessary constituents of 

 the soil. A plant with double the surface of another plant must 

 evaporate twice the quantity of water that the latter does. The 

 water thus absorbed is expelled again in vapor, but the salts and 

 constituents of the soil introduced to the plant by its agency, 

 still remain there. A plant with twice the surface of leaves of 

 another plant, but with the same quantity of water in proportion 

 to its size, still receives from the same soil a greater quantity 

 of ingredients, in proportion to its water, than the latter plant 

 receives. 



The growth of the latter soon reaches a termination when the 

 further supply ceases, while the former continues to grow, be- 

 cause it contains a larger quantity of the substances necessary 

 for the assimilation of atmospheric food. But in both plants the 

 number and size of the seeds will altogether depend upon the 

 amount of the mineral ingredients of the seed existing in the 

 plants ; the plants containing or receiving from the soil a greater 

 amount of alkaline and earthy phosphates than other plants obtain 

 in the same time, will also produce a greater number of seeds 

 than the latter. 



Thus it is that, in a hot summer, when the supply of the con- 

 stituents of the soil is cut off by rain, the height and strength of 

 the plants, and the development of the seed, stand in exact pro- 

 portion to the quantity of the constituents of the soil taken up 

 during their former period of growth. 



The produce of a field in corn and in straw varies very con- 

 siderably in different years. In one year we may obtain the 

 same weight of corn of similar composition to that obtained in 

 another year, but the crop of straw may be considerably greater j 



