ABSOEPTIOX OF RAIN AND DEW BY THE FOLIAGE-LEAVES. 225 



into a moulilering mass. But as soon as tliey perish other new air-roots, covered 

 with golden-browu velvet, make their appearance and take their place. These aerial 

 roots never reach the ground or adhere to any substratum, so that their hairs 

 ftinnot contract an organic connection with a solid body. It is consequently also 

 impossible in this case for the root-hairs to draw moisture from the soil in the 

 capacity of absorption-cells. 



These root-hairs, however, are scarcely ever in a position to take up even the 

 atmospheric deposits. The various species of Philodendron and the other epiphytes 

 referred to, have large leaves which cover the air-roots hanging from the stem like 

 umbrellas, and every tree-fern also bears at the top of its stem a tuft of great 

 fronds, which prevents falling rain from wetting the aerial roots. Moreover, the 

 very plants whose air-roots exhibit a velvety coating occur in woods where the 

 tops of the trees arch over the ground in lofty domes, and form a sheltering roof 

 against deposits from the atmosphere. On the other hand, the air within these 

 forests is saturated with aqueous vapour, and it is certain that the velvety roots 

 have the power of condensing vapour, and that the root-hairs instantly suck up the 

 condensed water and convey it to the deeper-lying layers of cells. The truth of 

 this has been established by the results of repeated experiments. Thus, air-roots of 

 the tree-fern Todea barbata, after being transferred from moderately damp air 

 into a chamber full of vapour, condensed and absorbed in the space of twenty-four 

 hours water amounting to 6i per cent of their weight. There is, therefore, no doubt 

 that water may be acquired in this way also by plants, even though the instances 

 may not be very numerous. All plants in which this kind of water-absorption has 

 been hitherto observed grow in places where the air is very moist the whole year 

 round, and where there is also no risk of the temperature falling below freezing- 

 point. Under other conditions, especially in places where the air is periodically 

 very dry, these plants would not be able to survive; for, although they possess 

 organs for the condensation and absorption of water, they have no means of protec- 

 tion against the desiccation of these organs. 



ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES. 



The idea that plants absorb with their roots such water as they require is so 

 intimately associated with our whole conception of plant-life, that tliis i^roccss is 

 commonly adduced for the purpose of analogies of the most various kinds, and one 

 looks upon the water-absorption efi'ected by aerial roots in the manner just described 

 really as a thing to be expected, notwithstanding the fact that in this case, as the 

 above account shows, the phenomenon is not so simple as is usually supposed. We 

 now turn to the considei-ation of land-plants. If the leaves of plants cultivated in 

 pots become flaccid, water is poured as quickly as possible upon the dry soil with a 

 view of supplying the roots which ramify in it with moisture. Nor does the result 

 fail to be produced. In a short time the foliage becomes fresh and elastic again, 

 the roots having discharged their function. Even in the open air, it is especially 



Vol. I. 16 



