2o6 THE PHYSIOLOGY OF PLANTS 



rally on the same lines as the evaporation of water from an 

 inanimate body will be understood if we reflect that the 

 factors which increase the evaporation in the latter case 

 (especially the rise of temperature) increase the vital activity 

 of the vegetable organism. An increase of the vital processes, 

 however, entails an increase of transpiration. On the other 

 hand, some conditions may occur under which tlie amount of 

 water given off by plants decreases, while inanimate bodies 

 would show an increase of transpiration. Thus sickly plants 

 may show a diminution of transpiration with a rising tempera- 

 ture. Inversely, however, an increase of transpiration will 

 be manifest even with a fall of temperature if plants are trans- 

 ported from a strongly concentrated nutritive solution to a 

 more dilute one. The roots will then appear to put forward 

 their best energies to absorb from the weaker solution as much 

 of the inorganic food matter as they formerly obtained from the 

 more concentrated solution. But this is only possible when in 

 the same interval of time larger quantities of the water are 

 taken up, which means, of course, that a greater amount must 

 be given off too. 



The fact that sickly plants are not able to give off as much 

 water as healthy ones must not be forgotten when plants with 

 injured roots are re-potted and placed in a hot-bed. It is then 

 often supposed that the increased temperature of soil and air 

 will stimulate the plant to increased transpiration. The pot 

 is therefore often kept continuously wet, and the decay of the 

 roots, against which one is fighting, will continue unabated. 



The amount of water necessary for plants is determined — 



(1.) By the kind of plant under eonsideration. 



(2.) By the intensity of the vital processes at any given 

 moment. 



To know the requirements of any special kind of plant, it 

 is necessary to know something of the conditions under which 

 the plant grows in its native country. As far as practical ex- 

 perience has taught us at present, we may assume that plants 

 from rocky habitats, without shade, and from countries with 

 long periods of drought, are able to do with a very small 

 supply of water, and can only put up with an occasional 

 drenching. In those cases in which the climatic condition of 



