Lecture XII. 93 



This directive action of chemical stimuli is termed chemotropism 

 or chemotaxis. The different behaviour of the vertical, creeping 

 and penetrating branches must, in the first instance, be due to 

 their difference in response to similar stimuli. Consequently, 

 although we can discover no microscopic difference in the proto- 

 plasm of these organs, there must be some physiological difference 

 which is responsible for their different behaviour. The emergence 

 of the vertical branches appears to be due to negative hydrotropism, 

 i.e. a response which causes them to grow from a region where 

 the concentration of water vapour is great towards one in which 

 the concentration is less. Once the vertical branch has emerged 

 the most important directing stimuli appear to be gravity and light. 

 Under usual conditions both stimuli act, and consequently the 

 branch takes up an intermediate position due to two responses. 

 If the branch is removed from the action of one, say of light, and 

 is grown in the dark, its response to gravity may be studied alone. 

 It will then be found that the vertical branches are negatively 

 geotropic, i.e. tend to curve until their growing tip is directed 

 away from the centre of gravity of the earth. This causes these 

 branches under normal conditions to assume and maintain a vertical 

 position. The branch will curve in this way as often as it is dis- 

 turbed from the vertical position. If the piece of bread from 

 which several vertical branches are emerging be set on its side, so 

 that the surface which was before horizontal becomes vertical, under 

 the new conditions the emerging branches will slowly bend through 

 90 and bring their tips once more into a vertical position. 



The mechanism of this movement is quite obscure. The 

 rigidity of the branch in the vertical position is due to the osmotic 

 pressure of the solutes in the vacuole pressing out the lining of 

 cytoplasm and putting the cell-wall in tension. The solutes re- 

 present the compression member and the cell-wall the tension 

 member in this rigid structure. If the member in compression be 

 removed as we may remove it by rendering the cytoplasm per- 

 meable by heat the branch will collapse. This internal pressure 

 is apparently also an important factor in the growth of the branch ; 

 it pushes the terminal part forward where the cell-wall is most 

 ductile. Hence we may assume that the unequal growth of 

 curvature is produced by one side of the wall of the growing region 

 being more ductile than the other and yielding more readily to the 

 internal pressure. This softening of the cell-wall may very prob- 

 ably be caused by the action of an enzyme made more effective on 

 one side by the accumulation of some substance acting as a co- 

 enzyme. The accumulation of hydrogen ions has been suggested. 



