DIA-HELIOTROPISM AND DIA-GEOTROPISxM 645 



different flanks is the same; but this, as will appear from 

 certain experiments to be described later, cannot be said of 

 dorsi-ventral organs, for in these geotropic sensitiveness is 

 different in the upper and lower halves. In my own experi- 

 ments on the action of light, however, I shall be able to give 

 results which are but little affected by geotropism. 



3. Effect of suctional activity and of turgescence. — There 

 are two other factors not hitherto taken into account which 

 exert considerable influence in determining the attitude 

 finally assumed by the leaf. These are the general condition 

 of turgescence of the plant, and the limits of flexibility 

 which characterise the particular responding organ. The 

 first of these is, as we have seen, dependent on the suctional 

 activity. Now, the mechanical response of a plant-organ is 

 the result, as we already know, of the expulsion of water 

 from the excited tissue ; but if the tissue be over-turgid 

 expulsion of water is opposed, and the responsive movement 

 is thereby reduced or abolished, as we have seen in the case 

 of Mimosa in a condition of excessive turgor. The same 

 phenomenon I have again seen manifested, in a remarkable 

 manner, in the difference of the movements made by the 

 leaves in response to light according as they were normally- 

 or super- turgid. Thus, in a small plant of Artocarpus, 

 grown in a pot, and in the autumn season, when the suctional 

 activity was not great, the leaves responded to light by 

 making a progressive angle with the vertical, so that, under 

 the long-continued action of light, they first reached the 

 horizontal position and then fell many degrees below it ; but 

 in the rainy season, when they held themselves abruptly 

 vertical in consequence of excessive turgor, the action of 

 light produced little or no responsive movement. In the 

 autumn season, again, the limited system of roots and rootlets 

 possessed by this plant, when grown in a pot, allows it only 

 a moderate degree of turgescence, and in this condition it 

 readily responds to light ; but a large tree of the same 

 species, growing in the open and possessed of a highly 

 ramified and extensive root-system, will during the same 



