360 THE EFFECT OF WATER UPON GROWTH [Cn. XII 



opposite directions. Thus WOKTMANN ('81, p. 374) finds that 

 a mycelium growing downwards towards water turns horizon- 

 tally before touching it and branches profusely. Similarly a 

 root growing towards water will not penetrate into it, but will 

 turn to one side. The greatly increased moisture causes the 

 reversal of the tropism, but this is probably due to the fact 

 that a false hydrotropism replaces the true response ; however, 

 as true hydrotaxis may take place in both directions, so there 

 may be a true negative as well as positive hydrotropism. 



I now summarize our conclusions concerning the effect of 

 water upon growth. Water plays a part in growth second in 

 importance to no other agent, so that in its absence growth 

 cannot occur. As the quantity is increased, growth is increased 

 until an optimum is reached. The amount imbibed does not, 

 however, depend directly upon the amount available, but rather 

 upon the needs or the habits of the species. Growth of elon- 

 gated organs may take place from or towards moisture, and the 

 turning may be a true response to the stimulus of higher or 

 lower aqueous tension, a response which may show itself in 

 a bending at some distance behind the irritable tip. This 

 response is, moreover, often of an advantageous kind, directing 

 the rootlets towards water and the pollen-tube towards the 

 moist stigma or keeping the sporangium in the dry atmosphere 

 necessary for the production of dry spores. In a word, imbi- 

 bition of water and growth with reference to the source of 

 moisture are regulated to the advantage of the species. 



LITERATURE 



DARWIN, C. '80. The Power of Movement in Plants. London, 1880. 

 DARWIN, F. '93. On the Growth of the Fruit of Cucurbita. Ann. of Bot. 



VII, 459-487. Pis. XXII, XXIII. Dec. 1893. 

 DETLEFSEN, E. '82. Ueber die von Ch. Darwin behauptete Gehirnfunction 



der Wurzelspitze. Arb. a. d. bot. Inst. Wiirzburg. II, 627-647. 

 DUCHARTRE, P. '56. Influence de 1'humidite sur la direction des racines. 



Bull. Sqc. bot. France. Ill, 583-691. 

 ERRARA, L. '93. On the Cause of Physiological Action at a Distance. 



Kept. Brit. Ass. Adv. Sci. for 1892, 746, 747. 



