IRRITABILITY. 465 



growing slowly or not at all, it is probably due to the shorten- 

 ing of the side which becomes concave. It appears, namely, 

 that just as we may have heliotropic curvature without growth 

 in length (p. 436), so we may have geotropic curvature. 

 Kirchner observed geotropic curvature in roots of Peas and 

 Beans, at a temperature of 2 3-5 C, a temperature at which 

 their growth in length must have been very slow if it took 

 place at all. When the curvature is distinctly accompanied 

 by growth in length, the rate of growth of the convex side is 

 greater than the mean rate of growth of the whole organ, 

 whereas that of the concave side is less. The relation be- 

 tween the rate of growth of the convex and concave surfaces 

 of a geotropically curving organ, as well as the relation of the 

 rate of growth of the organ as a whole to that of a similar 

 organ growing in the normal direction is well illustrated by 

 Sachs' observations on the roots of Vicia Faba, an example 

 of which is given below. 



One seedling was placed with its root vertical, and another similar 

 seedling was placed with its root horizontal : each root was marked out 

 into lengths of 2 m.m. each. At the end of 14 hours the four apical 

 lengths of the horizontal root (t. e. a portion 8 m.m. long) had grown and 

 become curved. 



Increment of length of convex side ... io'8 m.m. 



concave 6'i 



Mean increment 84 



The corresponding portion of the vertical root had grown in the same 

 time to the extent of 10*5 m.m. 



Comparing these results, and taking 10-5 as the normal increment of 

 growth of the root, we find that 



1. The increment of the convex side exceeds the normal by 0*3 m.m. 



2. concave side falls short of 44 



3. Mean curving root 2*1 



Sachs has arrived at the same result by comparing the length of the 

 cortical parenchymatous cells of the convex and concave sides of the 

 curving region of roots with that of the cells of the straight portion : the 

 numbers refer to divisions of the micrometer ; the measurements here 

 given refer to the root of Aesculus Hippocastanum. 



V. 3 



