SECT. II PHYSIOLOGY 191 



but also directly in that its elements, hydrogen and oxygen, are made 

 use of in the formation of organic compounds in plant nutrition. 

 Water is also necessary for the turgidity and consequent rigidity of 

 parenchymatous cells (p. 183); it is of use in the process of the 

 growth of plant cells, which take it up in large quantities, and, 

 by their consequent expansion, enlarge their volume with but little 

 expenditure of organic substance. 



Absorption of Water 



Aquatic plants and the lower land plants Avhich are but little 

 difFei-entiated, and also the Mosses, can absorb water by their general 

 surface. The same is true of many aquatic Phanerogams. These 

 (Utricidaria, Ceratopliyllum, JFolffia), like some Hymenophyllaceae of 

 damp primaeval forests, and the epiphytic Tillandsia iisneoides, often 

 possess no roots. — 



The case of a typical land plant is different; very simple exi^eriments 

 show that it obtains water from the earth by means of its roots. If 

 the soil is kept moist the plant flourishes, while separated branches 

 wilt and ultimately die, even in a moist atmosphere. It is true that 

 the above-ground parts are not completely impermeable to water, 

 but the cuticle renders its entrance so difficult that even submerging 

 the foliage does not suffice to meet the requirements of the plant for 

 water. The root, on the other hand, is specially adapted, both by its 

 external form and by the structure of its superficial layers, for the 

 absorption of water. The fact that in ordinary soils the water exists 

 in a condition of invisible and fine division, firmly held by the surface 

 forces of the particles of the soil, renders necessary a large exposure of 

 surface on the part of the absorbing root. This is obtained on the 

 one hand by the high degree of branching of the root, and on the other 

 by the development of root -hairs on the epidermis. These are 

 developed very close together, and stand straight out all round the 

 surface of the root ; they penetrate the narrowest crevices of the soil, 

 and fasten themselves so closely to its smallest particles as to seem 

 actually grown to them (cf. Fig. 179). Although they have the 

 diameter of only a medium-sized cell, and appear to the naked eye as 

 fine, scarcely visible, glistening lines, they often attain a length of 

 several millimetres, and greatly enlarge the absorbing surface of their 

 parent root. According to F. ScHWARZ the epidermal surface of the 

 piliferous zone of the roots of Fismn, which has 230 root-hairs to the 

 square millimetre, is thus increased twelvefold. The entrance of 

 water into the plant in a given time would be correspondingly 

 increased. 



The root-hairs cover only a comparatively small zone, a short 

 distance above the growing root-tip. Soon after they have attained 

 their greatest length, and have come into the closest contact with the 



