ABSORPTION AND MOVEMENT OF WATER 125 



iswamps, or it may be due to the actual difficulty of ab- 

 sorbing water.* 



SECRETION 



Before leaving the subject of the osmotic phenomena in the 

 plant-body to discuss those of water vaporization and gas 

 exchange, those osmotic . processes which result in the re- 

 moval of material from the plant should be mentioned. If 

 there is absorption by means of osmotic currents set up and 

 maintained because there are smaller proportions of water 

 and of various other substances inside than outside the cell, 

 there must be excretion by the same means whenever the 

 opposite is true. Such excretion does take place ; there are 

 exosmotic as well as endosmotic currents. They are very 

 different, however, in amount, rate, and character. By the 

 root-hairs of higher plants water and dissolved substances 

 are endosmotically absorbed. Minute quantities of a number 

 of substances are exosmotically excreted also by the root- 

 hairs. Since Sachs's classic experiment in growing roots 

 in contact with polished marble plates, t it has been known 

 that roots can exert a corrosive action on such solid mat- 

 ters as they touch. Recent experiments by CzapekJ have 

 shown that the principal substances diffusing from roots 

 are mainly carbon-dioxide (passing out as carbonic acid, 

 H.,CO 3 ), phosphoric, hydrochloric, sulphuric, and phormic 

 acids and their salts, preeminently substances which would 

 aid the plant to obtain needed food-materials from the soil. 



Besides the excretion from roots, the secretions in the 

 various glands and reservoirs are dependent upon exosmotic 



* Cowles, H. C. The ecological relations of the vegetation of the sand 

 dunes of Lake Michigan. Botanical Gazette, vol. 27, 1899. Schimper, A. 

 F. W. Pflanzengeographie auf phisiologischer Grundlage, 1898. 



t Sachs, J. von. Auflosung des Marmors durch Mais-Wurzeln. Bota- 

 nische Zeitung, 1860. Lectures on the Physiology of Plants, pp. 262, 

 263, Oxford, 1887. 



J Czapek, F. Zur Lehre von den Wurzelausscheidungen. Jahrb. f. wiss. 

 Botanik, Bd, 29, 1896. See also Sistini in Atti di Soc. Tosc. di nat., 

 Proc. Verb., 1899 (reviewed in Just's Jahresbericht, Bd. 27, 2te Abth., 

 p. 194), who says roots convert feldspar into clay, working at least 

 four times as fast as the weather. 



