MOVEMENTS OF VARIATION 32 1 



2. Paratonic growth movements (the tropisms, due to external stimuli). 



(a) Phototropic bendings. 



(b) Geotropic bendings. 



(c) Bendings due to other tropisms (hydrotropism, chemotropism, trauma- 



tropism, thigmotropism, etc.). 

 II. Movements of variation, or turgidity movements (in mature or maturing organs) . 



1. Autonomic movements of variation (due to internal stimuli). 



Movements of the lateral leaflets of the telegraph plant, Desmodium gyrans. 



2. Paratonic movements of variation (due to external stimuli). 



(a) Stomatal movements, opening and closing of the stomatal pores (due to 



changes in light intensity). 



(b) Photeolic movements of leaves of legumes, Oxalis, and some Euphorbias 



(often called "sleep" movements; due to changes in light intensity). 



(c) Contact, etc., movements of sensitive plants (as Mimosa piidica). 



2. Autonomic Movements of Variation. — The most striking example of autonomic 

 movements of variation is found in the movements of the lateral leaflets of the telegraph 

 plant, which move in such a way that the tip of each leaflet transcribes an ellipse. The 

 movement is very rapid and quite noticeable with high summer temperatures. The 

 terminal leaflet of red clover moves in somewhat the same way, but much more slowly. 



3. Paratonic Movements of Variation. — The contact movements of the leaves of 

 the sensitive plant are paratonic movements of variation, as also are stomatal move- 

 ments and the night-day movements of leaves of the legumes, etc. In the contact 

 movements and night-day movements, bending is not dependent on enlargement but 

 is due to a rapid change in the turgidity of cells of the leaf pulvini, which are special 

 bending organs. Upon being stimulated, the protoplasm of the active cells quickly 

 increases its permeability toward the osmotic substances dissolved in the vacuoles, 

 some of the cell sap moves into the intercellular spaces, and the tissue becomes flaccid. 

 After a time the sap is absorbed again and turgidity is regained. Some other plants 

 exhibit movements similar to those of the sensitive plant, but to a less marked degree. 

 The staminal filaments of some flowers show pronounced contact responses of a similar 

 nature. 



The night-day (photeolic) movements of the leaves of many plant forms (leguminous 

 plants, Oxalis, some Euphorbias) are brought about by pulvinus reactions much like 

 those described above, but in these cases the stimulus is not mechanical contact, but 

 a decrease or increase in light intensity. 



21 



