SECT. IV. ELABORATION OF THE SAP. 13Q 



power of the leaves, in the proportion of five to two. 

 Similar experiments were also made upon some 

 species of cabbage, whose mean transpiration was 

 found to be lib. 3oz. per day; and on some species 

 of evergreens, which were found, however, to tran- 

 spire less than other plants. The same is the case 

 also with succulent plants, which transpire but little 

 in proportion to their mass, and which as they 

 become more firm transpire less. It is known, 

 however, that they absorb a great deal of moisture, 

 though they give it out thus sparingly ; which we 

 cannot but regard as a wise institution in nature for 

 the purpose of resisting the great droughts to which 

 they are generally exposed, inhabiting, as they do for 

 the most part, the sandy desert or the sunny rock. 

 Along with his own experiments Hales relates 

 also some others that were made by Mr. Millar, 

 of Chelsea ; the result of which was that, other cir- 

 cumstances being the same, transpiration is in pro- 

 portion to the transpiring surfaces ; and is affected 

 by the temperature of the air, sunshine, or drought 

 promoting it, and cold and wet diminishing or 

 suppressing it entirely. It is also greatest from 

 six o'clock in the morning till noon, and is least 

 during the night. But when transpiration becomes 

 too abundant owing to excess of heat or drought, 

 the plant immediately suffers and begins to lan- 

 guish ; and hence the leaves droop during the day, 

 though they are again revived during the night. 

 For the same or for a similar reason, transpiration 



