IRRITABILITY. 587 



We will briefly discuss from this point of view the fact 

 mentioned above that an incision into the stem of Mimosa, so 

 soon as the fibrovascular tissue is touched, causes a movement 

 in the nearest leaf. We can now account for this by attribut- 

 ing it to a stimulation of the protoplasm of the cells of the 

 bast-parenchyma, the stimulus being transmitted in this tissue 

 to the nearest leaf. 



It must not, however, be assumed that the property of 

 conductivity is confined to the bast-parenchyma. From 

 Darwin's observations on Drosera and Dionsea, it appears 

 that stimuli are transmitted through the parenchymatous 

 cells of the leaf, and this must be also the case, as Cohn 

 points out, in that of Aldrovanda, in the lamina of which no 

 fibrovascular tissue is present. But Darwin has pointed out 

 that in Drosera the rate of transmission of the stimulus is 

 materially affected by the form of the cells and by the direc- 

 tion of their axes. A stimulus travels most rapidly in a 

 tissue consisting of elongated cells, the direction of transmis- 

 sion being the same as that of the long axes of the cells. 

 Clearly, in the case of Mimosa, the bast-parenchyma is, of all 

 others, the tissue which meets the mechanical requirements of 

 rapid transmission. It is not altogether clear why the rate of 

 transmission of a stimulus should be thus dependent upon the 

 form of the cells and the relation of their axes to the direction 

 of transmission ; but it may be, as Darwin suggests, that the 

 transmission is retarded on passing through cell-walls. This 

 suggestion at least enables us to account for the relatively 

 great rapidity of transmission through a tissue consisting of 

 elongated cells in the direction of their axes, for, under these 

 circumstances the number of cell-walls to be traversed is the 

 smallest possible. 



Biological Significance of Plant-Movements. 



In previous lectures we have described the phenomena of 

 movement presented by plants, and in the present lecture we 

 have endeavoured to explain the mechanism of movement. 

 In order to conclude the subject we must enquire, though we 



