2o8 THE PHYSIOLOGY OF PLANTS 



takes place very slowly, and is preceded by a long death-like 

 rigor, which is only overcome by a renewed supply of oxygen. 

 If death were to ensue rapidly in these cases, we should see 

 even a greater number of pot plants die than are actually 

 killed at present by too much watering. 



We need only observe the tremendous variations in the 

 amount of water supply to which these plants are exposed, 

 when they are dependent upon the rain, to see how little harm 

 these variations do. We are, therefore, generally in danger 

 of injuring the plants by a too anxious endeavour to supply 

 every small amount of water lost by the plant. Indeed, it is 

 very good for plants, just as for human beings, to feel the 

 passing pangs of hungei* or thirst, and they take no harm if 

 they are not watered until the pot sounds hollow. If water 

 is then supplied, it should be done so copiously that it runs 

 out of the hole at the bottom of the pot. The practice of in- 

 experienced persons of repeatedly wetting the upper surface of 

 the- soil (especially of plants in peaty soil) has the disadvantage 

 that the lower portion of the soil remains dry while the 

 surface and the base of the stem become covered with moss. 

 The above rule is especially important in the case of tough- 

 leaved plants, which do not require much moisture. It is 

 different in the case of herbaceous plants which are rapidly 

 growing. In their case every diminution of the water supply 

 is injurious, and in the case of these rapidly-growing plants 

 there is very little danger that the soil will be water-logged for 

 any considerable time, and consequently stop the respiration of 

 the roots. 



But even if we know the natural conditions of the plant and 

 the requirements of the species, we will not be able to water 

 the plant rationally if we do not know or do not consider the 

 peculiar requirements of every individual. Even the same 

 individual with the same amount of leafy tissue may need 

 a larger or a smaller amount of water according to the vital 

 activity which it manifests at any given moment. 



This difference in the amount of water required at different 

 periods becomes more comprehensible if we remember that the 

 water is not only needed as a food substance, the elements of 

 which become separated in the vegetable tissues, and used up 



