MOVEMENTS OF PLANTS. 197 



apices describe nearly a circle. A revolution occupies from 

 two to five minutes if the temperature is above 22 Cent. 

 (72 Fahr.). This continues, when the conditions are other- 

 wise favorable, in darkness as well as in the light. Other less 

 noticeable movements of this nature occur in many plants 

 e.g., Clover, Mimosa, Oxalis but they are often hidden by the 

 more marked movements due to other causes. The active 

 portion of the moving organ (in the cases cited above, a por- 

 tion of the leaf-stalk) consists of a tissue composed of thin- 

 walled cells, forming, in many cases, a thickened "pulvinus." 

 The cells are turgid and the tissues are in a state of tension. 

 When movements occur, it appears that the protoplasm in 

 certain layers of cells permits the escape into the intercellu- 

 lar spaces of a portion of the water of the vacuoles ; it is, 

 however, quickly absorbed again and the cells rendered 

 thereby turgid, while the escape of water takes place in 

 contiguous layers, to be quickly absorbed again, and so on 

 regularly around the axis of the contracting organ. 



259. Movements Dependent upon External Stimuli. 

 These are exhibited by many parts of the higher plants e.g., 

 leaves in Mimosa (the Sensitive Plant), Cassia, Clover, 

 Oxalis, Dionaea, etc., stamens of many Compositae, of Bar- 

 berry, Portulaca, etc., stigmas of Martynia, Mimulus, etc. 

 In the Sensitive Plant, the leaves, when touched roughly or 

 jarred, close up quickly by the secondary leaflets moving 

 upward and forward, so that the upper surfaces of the 

 pairs are approximated to each other ; next, the primary 

 leaflets bend downward, and at the same time approach each 

 other, and finally the whole leaf bends downward. The 

 movements are in all cases at the bases of the organs, where 

 tissues are developed similar to those in the spontaneously 

 moving organs (paragraph 258). In the other cases essen- 

 tially the same movements and mechanism are found. When 

 the movements occur, there is an escape of the water of the 

 vacuoles from the cells in one side of the organ, and this 

 side is, as a consequence, shortened and made concave. 

 After a time the water is reabsorbed and the organ resumes 

 its normal position. In addition to the mechanical stimuli 

 of jarring, concussion, etc., greater or less amounts of light, 



