194 BOTANY. 



Creeper (Ampelopsis) are negatively heliotropic, and they 

 are thus enabled to reach and attach themselves to the sur- 

 faces e.g., walls, tree-trunks, etc. which give them sup- 

 port. The same organ may be positively heliotropic in one 

 stage of its growth and negatively so in another ; thus the 

 younger internodes of the ivy (Hedera) bend toward the 

 light, and the older ones away from it ; and the runners of 

 Saxifraga sarmentosa, mentioned above, are positively he- 

 liotropic as soon as they develop tufts of leaves upon their 

 free extremities. 



The rays of light which cause the curvature are those 

 having the greatest refrangibility. Sachs' experiment shows 

 this conclusively ; he grew plants in light which had passed, 

 on the one hand, through a solution of potassium bichro- 

 mate, and, on the other, through one of ammoniacal copper 

 oxide ; in the light passed through the first solution (red, 

 orange, and yellow rays, and a portion of the green) there 

 was no curvature whatever, while in the blue, indigo, and 

 violet rays passed through the second solution the heliotro- 

 pic curvature was strongly shown. 



IV. GEOTKOPISM. 



255. Nearly all organs of plants have a definite, normal 

 direction of growth, which is in general terms, either toward 

 or away from the earth. Thus the plasmodium of Fuligo 

 varians creeps upAvard ; the conidia-bearing hyphse of moulds 

 grow upward, while the root-like hyphae grow downward ; the 

 stems of many mosses grow upward, and their rhizoids down- 

 ward ; in the higher plants the stems, as a rule, grow upward, 

 some root-stocks and other stems growing downward, how- 

 ever, while the roots, as a rule, grow downward. To these 

 phenomena of growth the name Geotropism* has been 

 given ; when the direction of growth is downward, the organ 

 is said to be positively geotropic, when upward, negatively 

 geotropic. 



Knight long ago proved gravitation to be the cause of 



* From tLe Greek yi), yen, the earth, and rpt-irciv, to turn. 



