-1859] WRITINGS OF JOSEPH HENRY, 185 



We can scarcely doubt that by this arrangement the mo- 

 tive power which gives rise to the circulation of the sap is 

 the heat derived from the atmosphere and the direct rays of 

 the sun. But a small part however of the material of which 

 the plant is mainly built up, (namely carbon) is elevated from 

 the roots. This is furnished, as we have before stated, by 

 the de-composition of the carbonic acid absorbed from the 

 atmosphere into the pores of the leaves, and there resolved 

 by the chemical ray of the sun. It is at this place that the 

 liquid brought up by evaporation is elaborated into true sap, 

 under the principle of vitality, which being carried down- 

 ward through the cells by endosmose, serves by secretion 

 to build up new cells, and thus to increase every part of 

 the plant. The rapidity of evaporation will depend, the 

 amount of heat being the same, upon the quantity of vapor 

 already in the atmosphere ; and hence with the same de- 

 gree of temperature the amount of work performed would 

 appear to be greater in a dry than in a moist atmosphere; 

 but since the carbonic acid which is decomposed is probably 

 absorbed by the water in the leaf, too rapid an evaporation 

 will retard rather than increase the useful effect. 



But little is known of the minutiae of this process, or how 

 far the results may be influenced by other causes than those 

 actually observed. We are assured however by observation, 

 that beyond a certain degree of heat, a given plant cannot 

 have a healthy condition, and also below a certain tempera- 

 ture, which is still above freezing, the sap of plants ceases to 

 have an active if any circulation. 



Heat necessary for the growth of plants. — The hypothesis was 

 early advanced that for each plant a certain amount of heat 

 is requisite in order to its developement from one stage of 

 growth to another; for example, in the case oi wheat, from the 

 time it begins to sprout until it arrives at its full maturity, 

 a definite quantity of heat is required, other conditions be- 

 ing the same, though the time in which it may be furnished 

 may be different in different instances. Different methods 

 however have been proposed for estimating this heat. Reau- 

 mur, who first advanced the hypothesis of the definite amount 



