236 ABSORBING SYSTEM 



this case, concerned with a tissue which simultaneously performs several 

 different functions (viz. absorption, storage and conduction of water). 

 It might, accordingly, have been dealt with in a later chapter ; on 

 account of its histological features, however, it is most naturally 

 associated with the foliar absorbing tissue of the Leucobryaceae. 



D. THE WATER-ABSORBING HAIRS OF FOLIAGE LEAVES. 124 



Not only the Bryophyta and certain Ferns (Hymenophyllaceae), 

 but also many Phanerogams, possess the capacity of absorbing water 

 through the surface of their foliage-leaves. If withered leaves or leafy 

 shoots are immersed in water the cut surfaces being kept dry they, 

 as a rule, regain their turgescence completely after a certain time, 

 and sometimes even in the course of a few hours. While the water- 

 absorbing capacity, which many leaves reveal under these conditions, 

 may acquire some ecological importance in certain circumstances, it 

 does not necessarily constitute a specially evolved useful adaptation ; 

 for every article that can be wetted at all is in some degree pervious 

 to water, and the living cells of a withering leaf will thus in any case 

 absorb water deposited upon the outside of the epidermal wall to a 

 certain extent, by virtue of their osmotic activity. Like transpiration, 

 this superficial absorption is primarily a physical process, whereas its 

 regulation in the interests of the uninterrupted normal activity of the 

 foliage leaf is effected by means of special adaptations, which come 

 into being only in relation to particular ecological conditions. It is 

 only in dry climates, or more generally under xerophytic conditions, 

 that plants find it necessary to make immediate use by superficial 

 absorption of every drop of rain or dew that comes in contact with 

 their foliage. 



The first stage in the regulation of foliar water-absorption, whereby 

 this phenomenon ceases to be purely incidental and becomes an integral 

 part of the vital activity of the plant, consists in the fact that absorp- 

 tion no longer takes place over the entire surface of the leaf, but is 

 restricted to definite points of entrance, which, as a rule, exhibit some 

 evident histological specialisation. It sometimes happens that the 

 outer epidermal walls are more pervious to water over the veins ; this 

 condition may be looked upon as the first step towards the localisation 

 of foliar absorption. Almost always, however, it is some specialised 

 form of trichome that is entrusted with the task of absorbing water ; 

 such structures will henceforth be referred to simply under the name 

 of absorbing hairs (or scales). The absorption of water may also be 

 assigned to organs of a somewhat different type, which likewise 

 correspond morphologically to hairs or to modified epidermal cells ; 

 these are the so-called hydathodes, which are fully dealt with in a 



