ii8 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



and descend to a great depth. ^ Some plants in Hereroland, Acanthosicyos ^ 

 for example, possess a specially large root-system which serves to raise 

 the subterranean water that Hes very deep. Tall perennial herbs of the 

 Hungarian steppe are extraordinarily deep-rooted. 



A peculiar device for the absorption of water is met with in the North 

 African halfa-grass, Stipa tenacissima, the rhizome of which has peculiar 

 epidermal cells, whose function is to absorb water.^ 



ii. Epigeous Organs that absorb Water. 



In general, the epigeous parts of land-plants are not fitted to supply 

 the plant with water by absorption, since the more or less impermeable 

 cuticle of the epidermis will permit them to take in water only to a slight 

 extent.'* In ordinary phanerogams the amount of water that can be 

 taken in thus is insufficient to compensate for the loss by transpiration. 

 Exceptions to this are provided by lichens, mosses, and other thallo- 

 phytes, which can endure prolonged desiccation and can rapidly absorb 

 liquid water by their whole surface and may store it up ; ^ even aqueous 

 vapour can be withdrawn from the air by many of them. 



To many other land-plants living exposed to periodically extreme 

 drought it is of great importance that they should be able to seize the 

 moment, often fleeting, when water is available ; and, as a matter of 

 fact, there are devices enabling epigeous parts to absorb water with ease 

 and rapidity. 



Hairs that absorb water were shown by Volkens ^ to occur on certain 

 desert-plants, such as Diplotaxis Harra, Stachys aegyptiaca, and Convol- 

 vulus lanatus ; and by Schimper ' in certain epiphytes, including Tillandsia 

 and other Bromeliaceae. These hairs are not cuticularized at their base, 

 and it is at this point that the water enters. The subject has been investi- 

 gated by Mez,^ who regards some Bromeliaceae as adapted to absorb 

 dew, and others to absorb rain. The numerous white hairs of cacti 

 may subserve the same function.^ 



The same role has been ascribed to salt-glands, which Volkens '^^ dis- 

 covered in the form of characteristic hairs on the leaves of various desert- 

 plants, including Reaumuria hirtella, Tamarix, Cressa cretica, Frankenia 

 pulverulenta, and Statice aphylla. These glands excrete solutions of 

 hygroscopic salts (chlorides of sodium, calcium, and magnesium), which 

 solidify during the day and impart to the plant-parts a white or grey 

 appearance ; at night-time the salt deliquesces because of the increase 

 in atmospheric humidity, and the parts concerned again become green 

 and dotted with numerous drops of solution, even though there may 

 have been no deposit of dew. Volkens expresses the opinion that the 

 plants are thus enabled to absorb water. But Marloth " regards the 



^ Warming, 1891, 1907-09 (see illustration). 



" Schinz, 1893. * Trabut, 1888. 



Ganong, 1894 ; Wille, 1887 ; see Chapter VII. 



^ The case of Sphagnum is discussed in connexion with bogs. See Chapter XLIX. 



Volkens, 1887. ' Schimper, 1884. * Mez, 1904 a. 



^ As regards hairs which in temperate Europe are reputed to absorb water 

 reference should be made to Lundstrom, 1884 ; Wille, 1887, and Henslow ; and, as 

 regards the structure and function of hydathodes to Haberlandt, 1904; see p. loi. 



" Volkens, 1887. " Marloth, 1887 a. 



