LECTURE vn. 



of definitely observing the periodicity are very great. Unger 

 was the first to assert its existence, he found a maximum 

 between 12 and 2 p.m., and a minimum at night, but inas- 

 much as his experiments were made with plants in the open 

 air, his results cannot be regarded as at all conclusive. 



Sachs also made experiments with a view of settling this 

 point, but he failed to obtain any results which he could 

 regard as satisfactory. Baranetzky kept plants in darkness 

 for 24 hours with the other external conditions as constant as 

 possible : he found that during the hours of the night the loss 

 of water was much greater than during the hours of the day, 

 but he could not detect any periodical variation. He came to 

 the conclusion that there is no independent periodicity of 

 transpiration. 



The following table illustrates Baranetzky's experiments made on a 

 plant of Cucurbita in a pot. 



We have now to consider the physiological significance of 

 transpiration in the economy. As might be anticipated, its 

 significance is great and its effects far-reaching. Hales 

 recognised this when he said, " the motion of the sap is there- 

 by much accelerated, which in the heartless vegetable would 

 otherwise be very slow ; it having probably only a progressive 

 and not a circulating motion, as in animals." Thus, it affects 

 the absorption of water by the root, and in so doing it probably 

 promotes the absorption of salts in solution ; further, there is 



