502 



PLANT RESPONSE 



Fig. 209. Diagrammatic Representation of 

 Different Positions of a Single Cell, accord- 

 ing as the Specimen is held at an Angle of 

 45° or 1 35 , showing Consequent Redistribu- 

 tion of Statoliths (after F. Darwin) 



A, apical, B, basal, ends of cell. 



apical half. Some light may, perhaps, be thrown on this 

 subject from the results of my experiments on Biophytum. 

 I found that on subjecting this plant to the favourable tonic 



condition of rise of tem- 

 perature, the younger 

 leaflets began to show 

 spontaneous excitatory 

 response much earlier 

 than the older leaflets. 

 This shows that in an 

 excitable tissue the 

 younger portions are, 

 generally speaking, the 

 more sensitive. This 

 will probably account 

 for the difference of 

 geotropic reaction in 

 the case under consideration, for the apical halves of the cells 

 are relatively younger than the basal halves. 



Determination of the true character of apogeotropic 

 response. — I shall now proceed with the crucial determi- 

 nation of the true nature of gravitational response. In the 

 diagrammatic representation of the multicellular organ, we 

 have the upper and lower responding layers of cells repre- 

 sented by E and E' (fig. 204). Each of these may be one or 

 more layers in thickness. Since in apogeotropic organs the 

 curvature is upwards, this response may be due (1) to the 

 relative expansion or acceleration of growth of the lower side, 

 or (2) to the relative contraction or retardation of growth of 

 the upper side. As regards the first hypothesis, the curva- 

 ture induced in grass haulms has been assumed, as stated 

 before, to have proved that the gravitational response is one 

 of accelerated growth. On the other hand, the curvature 

 may be due to the active contraction of the upper side, the 

 response then being of the same nature as was seen demon- 

 strated by the irritating pressure of the magnetic particles. 

 The crucial experiment in deciding between the two alter- 



