250 MOVEMENT IN PLANTS* 



secreted by the parenchyma of the lower half of the pulvinus. 

 Moreover, at the moment of stimulation this part becomes dark 

 coloured, in the same way as a leaf that has been injected with 

 water under the receiver of an air-pump ; water has entered the 

 intercellular air-spaces. 



This excretion of water from the pulvinus cells might be due to 

 either (1) increase in the elasticity of the cell-walls, (2) decrease in 

 the osmotic pressure of the cells. At any rate, in the movement 

 there is a decrease in expansive power on the lower side of the 

 pulvinus ; expansion on the upper side is due simply to the removal 

 of the opposing pressure below, and the weight of the leaf helps 

 to compress the lower side. But that the weight of the leaf is not 

 necessary for the carrying out of the movement is readily seen ; 

 place a plant horizontally or vertically,, arid note that in both cases 

 the sensitive side of the pulvinus contracts in response to stimu- 

 lation. 



VIII. EXPERIMENTS WITH SUNDEW. 



351. Material for Study. Dig up a number of Sun- 

 dew plants, with the peat about their roots, and grow them 

 in pots of wet sphagnum. Note the arrangement of the 

 leaves in a rosette, the rounded or oblong form of the 

 blade, the tentacles borne on the slightly concave upper 

 surface of the blade the central tentacles erect and short- 

 stalked, those at the margin long-stalked and bent out- 

 wards. Examine a tentacle with the microscope, and note 

 that a vein runs up the centre of the stalk, ending in a 

 mass of spirally thickened cells in the centre of the glan- 

 dular head, the outer (epidermal) cells of which are 

 columnar in form. 



352. Responses to various Stimuli. The tentacles 

 react both to mechanical (contact) and chemical stimuli. 

 In their responses to mechanical stimuli the tentacles 

 agree closely with tendrils. 



(a] Strike a gland with a pencil : a single tap produces 

 no movement. Watch the tentacle for about a minute, 

 then tap it again several times, and note that movement 

 results. 



(6) Place on a gland a small particle of sand or gravel : 

 movement occurs. 



