186 PHYSIOLOGY OF NUTRITION. 



The plants wither under the conditions described, because the 

 roots are unable to take up this firmly retained water quickly 

 enough, and in sufficient quantity, to cover the loss by transpira- 

 tion. 1 



It is further instructive to remove from the flower-pot small 

 quantities of soil, a few grams in weight, after the plants have 

 withered considerably, and determine their behaviour when placed 

 in air containing much aqueous vapour, but not so moist that dew 

 will be deposited. It is best to place the weighed samples of soil 

 in small dishes, together with a vessel of water and a hygrometer, 

 under a box. The air in the box will obviously be very rich in 

 aqueous vapour, and the mercury in the dry-bulb thermometer of 

 the hygrometer will consequently stand only slightly higher than 

 that of the wet-bulb thermometer. In spite of the large quantity of 

 water in the air surrounding the samples of earth, it is found that 

 they do not condense any of the vapour, but, on the contrary, con- 

 tinue to lose water by evaporation. We know that thoroughly 

 dry earth is able to condense vapour, but this property of the soil 

 is of no importance for vegetation in general, since most plants, as 

 our experiments teach, perish when the soil still contains so much 

 water that its power of condensing aqueous vapour, even under 

 very favourable conditions, cannot yet be exhibited at all. 



1 See Detmer, in Wollny's Forschungen auf dem Gebiete der Agriculturphynk, 

 Bd. 1, and Journal f. Landwirthschqft, 27. Jabrgang. There is also given the 

 literature, and further features of the absorption of water by roots are discussed. 



70. Absorption of Water by the Leaves. 



The question of water absorption by leaves is not of great 

 physiological interest, but still it may be here briefly discussed. 

 In many leaves (Brassica, Zea, Aristolochia Sipho, etc.) on dipping 

 the lamina into pare water it appears to be covered by a silvery 

 layer, which is only interrupted along the course of the stronger 

 nerves. On removing the leaves from the water, we find that 

 only the cuticle over the nerves, and hairs which may be present, 

 have been wetted. The cuticle over the mesophyll, owing to its 

 more or less pronounced waxy character, cannot be wetted, and 

 hence remains dry even after contact with the water. The silvery 

 lustre referred to arises fronj. a layer of air between the leaf-tissue 

 and the water, whereby the light is totally reflected. If the leaves 



