ABSORPTION AND MOVEMENT OF WATER 141 



sap draws water from adjacent cells with corresponding 

 force. The decrease in the water-content of the parenchyma 

 cells causes a greater draft upon the water-conducting 

 tissues not only adjacent but throughout the conducting 

 system, for a draft upon one part disturbs the balance 

 throughout the whole. Thus, assuming an adequate force 

 by which water is raised through the ducts from root to 

 leaves, we see that this force is set in motion, and its action 

 is regulated, by the amount and the rate of transpiration. 

 Transpiration must then also affect the root-hairs, regulat- 

 ing the amount of water which they absorb. A current for 

 parts of its course osmotic, for the remainder of much 

 larger dimensions is set up, maintained and controlled by 

 transpiration. Transpiration is, however, only one means 

 of doing this, water-pores (p. 128) and hydathodes (p. 128) 

 being the others, and perhaps equally important for the 

 plants which possess them. 



The ordinary means by which evaporation is controlled by 

 higher plants are two: fiist, the stomata (p. 142), which 

 control the exchange of gases as well as of water-vapor be- 

 tween the plant and the air ; second, the character of the 

 epidermal and other cells (cork, etc.) covering the plant.* 

 Special means of facilitating or checking transpiration are 

 possessed by plants inhabiting especially damp or especially 

 dry regions, f 



* Consult De Bary, A. Comparative anatomy of the vegetative organs 

 of phanerogams and ferns. Oxford, 1884. 



t These topics need not be discussed here. They illustrate no new princi- 

 ples in plant physiology. The following papers may be read by the inter- 

 ested student. Other papers and books have been referred to in the pre- 

 ceding pages. Stahl, E. Regenfall und Blattgestalt. Ann. du Jardin Bot. 

 de Buitenzorg, Bd. XI., 1893. Uber bunte Laubblatter. Ibid., Bd. XIII., 

 1896. Kny, L. Zur physiologischen Bedeutung des Anthocyans. Atti del 

 Congresso Botanico Internationale, 1892. (Older literature here cited.) 

 Keeble, F. W. The hanging foliage of certain tropical trees. Annals of 

 Botany, vol. IX., 1895. Darwin, Charles and Francis. The Power of Move- 

 ment in Plants, 1880. Stahl, E. Uber den Pflanzenschlaf und verwandte 

 Erscheinungen. Botanische Zeitung, I. Abth., Heft. V.,VI., 1897. Wilson, 

 W. P. and Greenman, J. M. Preliminary observations on the movements 

 of the leaves of Afelilotus alba L. and other plants. Contrib. Botan. Lab- 

 oratory, Univ. of Pennsylvania, 1892. 



