104 LECTURE VII. 



is thus brought into relation with the air, and it is therefore 

 from them, as Hales first pointed out, that the greatest loss 

 of water takes place. Hence we may speak of them as the 

 transpiring organs of the plant. The following account of 

 this function may be assumed, unless it is expressly stated 

 otherwise, to apply to them. 



The first point which we will consider is as to the amount 

 of water which a plant may lose by transpiration in a given 

 time. This has been determined in various ways : by weigh- 

 ing the plant at stated intervals, as Hales did, or by col- 

 lecting the transpired water, as was done by Mariotte and 

 by Guettard, or again by observing the amount of water ab- 

 sorbed by the plant, a method which is due to Woodward. 

 Of these, the first two are the more reliable, the third being 

 open to the objection that the activity of absorption of either 

 the root of a plant or of the cut surface of a branch is not 

 necessarily a measure of its transpiration. 



The following are some of Hales's results, obtained by the method of 

 weighing : 



(a) Sunflower (Helianthus animus). 



Maximum loss in 12 hours (day) 30 oz. 



Mean 20 oz. = 34 cub. in. 



Loss during a night 3 oz. 



Area of leaf surface 5616 sq. in. 



Mean transpiration per sq. in. (day of 12 hrs.) y^ cub. in. 



(ff) Cabbage. 



Maximum loss in 12 hours (day) 25 oz. 



Mean 19 oz. = 327 cub. in. 



Area of leaf surface 2736 sq. in. 



Mean transpiration per sq. in. (day of 12 hrs.) ^ cub. in. 



These figures suffice to shew that the amount of aqueous 

 vapour which is exhaled by the leaves of a plant is very con- 

 siderable ; but it is less than that which would evaporate in 

 the same time from an equal surface of water, a fact which 

 was first ascertained by Hales and has since been more accu- 

 rately determined by Unger and by Sachs. 



