450 PHYSIOLOGY OF GROWTH. 



maintained that gravity does not modify the osmotic pressure of the 

 cells in the region of curvature, but modifies the power of resistance 

 of the turgor- stretched cell layers (protoplasm and cell membrane). 

 In negatively geotropic organs, the resistance of these layers in 

 the cells of the under side, which is becoming convex, is lessened ; 

 that of the cells of the upper side, which is becoming concave, is 

 increased. Consequently, the osmotic pressure remaining the 

 same, the turgor-expansion of the cells of the lower side will be 

 increased, that of the cells of the upper side diminished, and so 

 we have at once the conditions for a difference in the growth 

 of the opposite sides of the structure. Presumably, also, the 

 quantity of water in the lower half of a structure executing a 

 negative geotropic curvature will be greater than that of the 

 upper half, and indeed Kraus 4 asserts that he has demonstrated 

 such differences in the distribution of the water. Renewed re- 

 searches, however, are required concerning the distribution of 

 water in geotropically curved plant organs, since at any rate the 

 results obtained by Kraus respecting the distribution of water in 

 heliotropic stems have not been confirmed by Thate's observa- 

 tions. 5 



That in fact the osmotic pressure of the cells of the opposed 

 sides of geotropically curved organs is the same, as above indi- 

 cated, may be shown experimentally by the plasmolytic method. (i 

 Bean epicotyls grown in the dark are laid horizontally. If, after 

 upward curvature has taken place, we remove thin sections of the 

 cortex of both convex and concave sides, and examine microscopi- 

 cally, we shall observe the following. If the sections are laid in 

 2 per cent, solution of Potassium nitrate, no plasmolysis is as yet 

 exhibited. Plasmolysis only appears when we work with about 

 2'5 per cent, solution of Potassium nitrate, and a solution of par- 

 ticular concentration acts in the same manner on the cortical cells, 

 both of the convex lower and concave upper sides of the curved 

 epicotyl. 



If differences in osmotic pressure on the opposite sides of curved 

 plant organs are not to be regarded as the cause of the nutations, 

 the cause is probably to be sought in a special behaviour of the 

 membranes. "Wortmann 7 has endeavoured to support this view 

 by the following instructive experiment. A pot plant of Phaseolus 

 with an actively growing epicotyl is laid horizontally. Round the 

 tip of the stem is fastened a silk thread, which passes over a 

 lightly running pulley, and carries a weight sufficient to prevent 



