245 



the mechanical or molecuhir onorijy, that is at the disposition of the 

 plant, but it is also the last consideration to l)e studied and under- 

 stood. 



The temperature is the next important climatic element and that 

 which has been most studied; the heat involved in temperature is 

 the mechanical, molecular energy that is utilized by the vital powers 

 of the plant." Each plant utilizes a fraction of the molecular energy 

 that is at its disposition, according as its sunshine, temperature, and 

 sap are favorable to the formation of the chemical substances that it 

 can elaborate within its cells. The remaining elements important to 

 the production of crops are : 



(a) The water that enters the root, which may be natural rain or 

 artificial irrigation. 



(b) The chemicals dissolved in the water. 



(c) The soil that furnishes these chemicals. 



(d) The atmosphere that furnishes nitrogen, oxygen, and carbonic- 

 acid gas. 



(e) The evaporation of moisture from the plant and soil, mostly 

 through the influence of the wind and heat. 



Of these, only the rain water, the gases in the atmosphere, and the 

 evaporation are, properly speaking, meteorological or climatic ele- 

 ments not under the control of man; whereas the irrigation of the 

 soil and its chemical constitutents are largely under his control. 



The quantity of water actually consumed by the plant or evapo- 

 rated from its leaves and that which is daily evaporated from the soil 

 or which drains away to other localities, and thus becomes useless to 

 the plant, have been the subject of many experiments, some of whose 

 results may be summarized as follows : 



Thus, for example, Lawes and Gilbert, at Rothamsted, England, 

 from experiments in vases entirely under their control, derived the 

 following numbers, showing the weight of water evaporated relative 

 to the weight of grain produced per unit area of ground: 



In these experiments, therefore, the ground during the wheat sea- 

 son consumed water equivalent to a rainfall of from 184 to 212 milli- 

 meters in order to produce a harvest of 30 hectoliters, or 80 kilograms 

 in weight per hectare. 



Is it not in fact the vital power of the plant? — C. A. 



