PHYSIOLOGY 487 



leaf-base, consisting principally of parenchyma capable of great 

 changes in turgescence. In the ordinary position, the lower side of 

 the pulvinus has its cells more strongly distended than the upper 

 side, and there is pressure upon the lower side of the leaf-base which 

 pushes upward and holds the leaf in its horizontal or obliquely up- 

 right position. In its relaxed condition, induced by shock, absence 

 of light, or some other stimulus, there is a loss of water from the 

 lower cells of the pulvinus, which lose their turgor, and the leaf 

 sinks downward. 



External Stimuli 



In general, the conditions which govern the other activities of the 

 plant also affect its movements. Thus well-nourished plants, pro- 

 vided with the optimum temperature and illumination, will react 

 most vigorously in their movements. Beyond its general stimulat- 

 ing character, the temperature does not specially affect the character 

 of plant-movements. Up to a certain optimum temperature the 

 movements are accelerated, and beyond this they diminish, finally 

 ceasing entirely, a state of rigor being induced which ends in death 

 if the high temperature is long maintained. 



Chemical Stimuli 



Chemical stimuli must be of the greatest importance in the auto- 

 nomic movements of plants, and are also evident in certain induced 

 movements. Thus many low motile organisms are extremely sensi- 

 tive to certain chemical substances. Among these " chemotactic " 

 organisms are many Bacteria, which quickly accumulate at a point 

 where some special substance such as oxygen is being excreted. 

 Spermatozoids of Ferns are strongly attracted by salts of malic acid, 

 and will quickly swim into a capillary tube containing a dilute so- 

 lution of this substance, which it is assumed resembles that thrown 

 out by the open archegonium. Pollen-tubes are negatively aerotropic, 

 and they are also influenced in the direction of their growth by the 

 sugar} 7 secretions of the stigma and the cells of the conducting tissue 

 and ovule. The hyphae of Fungi are also strongly chemotropic. 

 Germinating spores, placed upon a perforated membrane (e.g. a bit 

 of epidermis with stomata) will grow through the openings, if the 

 membrane is placed over a chemically attractive substance, but will 

 not do so if the membrane is floated upon water, or upon an indif- 

 ferent fluid. 



Mechanical Stimuli 



Mechanical stimuli are of various kinds : pressure, shock, stretch- 

 ing, wounding, etc. To all of these there is a more or less evident 



