no 



LECTURE vn. 



In order to meet the objection that the increased transpiration under 

 the influence of light in these experiments is to be attributed to the 

 opening of the stomata, Wiesner states that in the case of the Maize the 

 stomata were nearly closed whilst the plant was exposed to light, and 

 that in Hartwegia comosa the stomata were wide open whilst the plant 

 was in darkness. He refers the more evident action of light in the 

 green Maize-plant, as compared with the etiolated one, to the fact that 

 the rays of light absorbed by the chlorophyll of the green plant are 

 converted into heat, a conversion which is not effected to the same extent 

 by the etiolin of the etiolated plant. 



In addition to the external influences already noticed, it 

 has been found that transpiration is considerably affected by 

 the nature of the liquids which are absorbed by the roots. 

 Senebier seems to have been the first to notice this, and he 

 endeavoured to investigate it by comparing the transpiration 

 of a number of branches with their cut surfaces in different 

 solutions. The investigation was subsequently resumed by 

 Sachs, and still more recently by Burgerstein. Sachs found 

 that a Tobacco-plant growing in coarse sand transpired less 

 actively than one growing in clay. In a series of water- 

 cultures, he found that when the roots of the plants (Cucurbita] 

 were in acidified water the transpiration was much greater, 

 whereas when they were in alkaline water the transpiration 

 was considerably less, than when they were in distilled water. 

 He also watered plants growing in pots (Gourds, Beans, 

 Tobacco) with I percent, solutions of potassium nitrate, ammo- 

 nium sulphate, and other salts, and found that the presence of 

 the salt diminished the transpiration. Burgerstein's results 

 agree in the main with those of Sachs', as the following 

 figures will shew; but he finds that some salts tend to in- 

 crease the transpiration. 



