136 BRITISH FLOWERING PLANTS cHAr. 



elastic cell -wall and its lining of living protoplasm. 

 Sucli an escape of water as is responsible for the move- 

 ment is rendered possible only by this property of the 

 protoplasmic lining undergoing some change, or, in 

 other words, by the hitherto non-permeable protoplasm 

 becoming permeable in consequence of the stimulus, 

 and thus letting water escape. It must at the same 



Fig. 79. — Transverse section of the motile organ of a leaflet of Oxalis carnea. 6, 

 axial vascular strand ; o, epidermis ; ep, parenchyma ; ch, inner layers of . 

 parenchyma ; s, sheath of vascular strand. 



time be added that we can at present form no idea why 

 this change in the protoplasm occurs in consequence of 

 a stimulus, or with what molecular changes it is con- 

 nected ; all we know at present is that the externally 

 perceptible effects of stimulation so far as described 

 are caused by the change referred to in the protoplasm 

 itself. 



The movements of the chlorophyll grains within 

 the cells were first noticed by Bohm. They have 

 been well described by Stahl. The central tissue of 



