THE ABSORPTION OF WATER 35 



Hellriegel. 1883. Beitr. z. d, naturw. Grundlagen des Ackerbaues. Braun- 

 schweig. 



HoHNEL. 1879. WoUny's Forsch. auf d. Geb. d. Agrikulturphysik, 2, 398. 



JosT. 1887. Bot. Ztg. 45, 601. 



EIarsten. 1892. Bibliotheca botanica, Heft 22. 



KiHLMANN. 1890. Pflanzenbiol, Studien aus Russisch Lappland. 



Kny. 1895. Ber. d. bot. Gesell. 13, 361. 



Kny. 1898. Ibid. 16, 216. 



KosAROFF. 1897. Einfl. auss. Faktoren auf d. Wasseraufnahme. Diss., Leipzig. 



Kraus, C. 1892. Wollny's Forsch. auf. d. Geb. d. Agrikulturphysik, 15. 



Mayer, Ad. 1901. Agrikulturchemie, 5th ed., II, i, 154. 



NoBBE. 1872. Landw. Versuchsstationen, 15, 391. 



Nobbe. 1875. Tharandter forstl. Jahrb. 201. 



Ramann, E. 1893. Forstl. Bodenkunde u. Standortslehre. Berlin. 



Rysselberghe. 1901. Bull. Acad. Belg. (Sciences), 1901, Nr. 3. (Recueil Inst, 

 bot. d. Brux. 5, 209.) 



Sachs, i860. Bot. Ztg. 18, 123. 



Sachs. 1865. Handb. d. Exp.-Physiol. (Hofmeister, Handb. d. phys. Bot. 4). Leipzig. 



Sachs. 1882, Vorlesungen iiber Pflanzenphysiologie. Leipzig. 



Schimper. 1888. Die epiphytische Vegetation Amerikas. Jena. 



Schumacher. 1867. Jahresb. f. Agrik.-Chemie, 83. 



Schwarz, F. 1883. Unters. aus d. bot. Inst. Tubingen, i, 135. 



Stahl. 1893. Annales Jard. Buitenzorg, 11, 98. 



Stahl. 1897. Bot. Ztg. 55, 71. 



Wacker. 1898. Jahrb. f. wiss. Bot. 32, 71. 



WiESNER. 1882. Sitzungsber. Wiener Akad. 86, 40. 



WoLLNY, 1897. Forsch. auf d. Geb. d. Agrikulturphysik, 20, 52, 



LECTURE IV 



TRANSPIRATION 



After this brief reference to epiphytes we may now return to the con- 

 sideration of ordinary land plants, of which our trees and cultivated plants 

 may be taken as examples. The soil, we have seen, supplies them with the 

 water they require, and that they absorb by the root ; we must now study 

 the reverse process, viz. the giving off of water by transpiration from parts 

 above ground, more especially from the leaves. 



No special methods are required to demonstrate this phenomenon, for 

 just as a free water surface, a sponge saturated with water or damp soil, gives 

 off water vapour into the air, provided the latter be not itself saturated, so 

 too must also the plant, containing as it normally does abundance of water. 

 And just as under natural conditions the water evaporated is not always re- 

 placed at once, so, too, transpiring plants exhibit great variations in the 

 amount of water they contain, variations often so obvious as to be noticeable to 

 the naked eye. Who has not noted herbaceous plants and even trees with limp 

 leaves or flowers on a hot day in July ? The wilting is simply the indication 

 of the suppression of osmotic distention of the cell-walls and, consequently, 

 of tissue tensions, owing to the loss of water. So long as the loss of water keeps 

 within certain limits, a renewed supply of water can once more induce normal 

 turgescence ; consequently, we notice that not infrequently during the night, 

 when transpiration is reduced by lowering of the temperature, the leaves again 

 become rigid. Not only from such everyday experiences, but also from the 

 fact that we can prevent wilting by placing the plant in the shade, or by water- 

 ing it at the right time, even the ' man in the street ' can appreciate the im- 

 portance of external conditions in determining the amount of transpiration. 

 Before passing to the consideration of the question as to how far the plants 

 themselves and how far external factors influence transpiration, we may glance 



D 2 



