466 LECTURE XVIII. 



Mean length of cells of straight portion of root 20- 1 



convex side 28*1 



concave side 9*3 



curved portion of root 187 



Difference i '4 



The action of gravity induces growth in organs which 

 have ceased to grow but which are still capable of growth, or 

 more rapid growth in organs which are growing but slowly, 

 provided that they are geotropically irritable. A case in 

 point is afforded by the haulms of Grasses, to which allusion 

 was made in a previous lecture (p. 333). After they have en- 

 tirely or nearly ceased growing in the normal vertical position, 

 they will, if laid horizontally, begin to grow again at the nodes 

 with considerable activity, the result being an upward cur- 

 vature. Elfving has compared the rate of growth of Grass- 

 haulms, which had nearly ceased growing, when in the vertical 

 position and when rotated horizontally on a clinostat. In one 

 set of observations he found that the mean increment in 

 44 hours was, for the vertical haulms 2*4 (micrometer-divisions), 

 and 6*4 for the haulms on the clinostat; in another set the 

 figures are respectively 1*3 and 11*3. The effect of the slow 

 rotation on the clinostat is that each side in turn tends to 

 become convex, that is, begins to grow more rapidly, and thus 

 the rate of growth of the whole haulm is increased. 



Growth, we know, depends upon turgescence ; hence the 

 geotropic curvatures of growing organs depend upon the 

 turgidity of their cells ; and since it is only living cells con- 

 taining protoplasm which can be turgid, it must be by such 

 cells of the organ that its curvature is effected. Such cells 

 constitute the parenchymatous tissue of the organ. It might 

 be thought that the pith plays an important part in pro- 

 ducing curvature, but it appears that this is not the case. 

 Sachs found, namely, that the pith of shoots, when freed from 

 the other tissues, cannot be induced to curve geotropically, 

 and de Vries found the same to be the case with the me- 

 dullary tissue of the nodes of Grass-haulms. Sachs has also 

 observed, in a shoot of Nicotiana Tabacum which had become 

 geotropically curved, that when the pith was isolated it at 



