CHAP, xxix ADAPTATIONS OF LAND-PLANTS 101 



by means of water-excreting organs, to which Haberlandt l has given the 

 name of hydathodes. They are possessed by land-plants as well as by 

 some water-plants, and cause water to be discharged by exudation- 

 pressure in the form of drops. Not only in the tropical rain-forest, but 

 also in the temperate countries, are there many plants, especially herbs, 

 exhibiting the phenomenon of guttation. When transpiration is depressed 

 by saturation of the atmosphere, there comes the danger that the plant, 

 on account of continued and powerful root-pressure, may take up an 

 excess of water from the wet soil, and thus attain a condition of maximal 

 turgescence when the air is expelled from the intercellular spaces and 

 completely replaced by water. The danger is averted by the hydathodes. 

 These organs mainly belong to the following types : 



1. Epidermal cells, sometimes of remarkable structure, or peculiar 

 hairs which are unicellular or multicellular, and in the latter case often 

 assume the form of glandular hairs. As these organs occur on both leaf- 

 faces, but particularly on the lower face, drops of water excreted over the 

 leaves simulate dew-drops. 



2. In some ferns hydathodes assume the form of peculiar glandular 

 spots on the lamina. 



3. The familiar water-pores, which are constructed like stomata, occur 

 on the upper face of leaf -teeth above a small-celled, thin-walled, usually 

 colourless tissue (epithema) in which the vascular bundles terminate. 



It must, however, be noted further that water may be excreted through 

 the epidermis by means of pores opening outwards, without the co-opera- 

 tion of hydathodes ; and also that water may be excreted without the 

 aid of living cells, for example in grasses : this contrasts with the preceding 

 cases in which living cells are the essential and functionally active organs. 



The adaptation of the land-plant to its existence in contact with air 

 proceeds on the following lines : 



1. Control of the outgo of water, i.e. regulation of transpi- 

 ration. 



2. Increase of the intake of water, i.e. development of special 

 mechanisms for absorption. 



3. Arrangement for storage of water, i.e. development of 

 water- reservoirs. 



In the following three chapters these adaptations will be discussed, 

 and in a succeeding one we shall deal with some structural characters and 

 growth-forms of land-plants, the utility of which to the plant is obscure, 

 although their connexion with its existence in a dry environment is 

 beyond question. 



At the outset it may be noted that the degree of adaptation of land- 

 plants to their life in contact with air varies widely, according as the 

 external conditions are more or less extreme. Those species that are 

 adapted to meet the conditions of strongest transpiration and most 

 precarious water-supply are termed xerophytes : z the remainder are termed 

 mesophytes : 3 between these two classes there is of course no strict 

 boundary. 



1 Haberlandt, 1894-5, 1904. 



1 Xerophyta, Schouw, 1822 (fjjpor, dry ; <f>vr6v, plant). 



* Mesophyta, Warming, 1895 (ptvos, middle). 



