240 BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 



those of tentacles, some stamens, and leaves of the Sensitive Plant are 

 due to alterations in tissue tensions independent of growth. 



581. Movements may be due : (1) to internal causes, or (2) to ex- 

 ternal influences. The first are spontaneous, the second induced. 



582. Spontaneous growth movements. — Darwin showed that the 

 tips of growing parts of plants — stems, leaves, roots — perpetually 

 move in irregular elliptical curves. Since the motion is one of bow- 

 ing toward all points of the compass in turn, he called it circumnutation. 



583. Induced growth movements. — These are much the more strik- 

 ing. The exciting causes {stimuli) are chiefly: gravity, light, mois- 

 ture, mechanical contact, and variations of light and heat. 



584. Gravity. — It has been observed from actual experiment in 

 the laboratory that roots of seedlings turn toward the center of the 

 earth, while the plumule turns toward the zenith. All turnings under 

 influence of gravitative force are manifestations of Geotropism. The 

 root is said to be positively, the shoot negatively, geotropic. 



585. Light. — Plants turn, as we say, instinctively toward the light. 

 If one could observe the root, however, it would be found to turn aw'ay 

 from light. These actions are instances of Heliotropism. The shoot 

 is, in general, positively heliotropic, the root negatively heliotropic. 



586. Moisture. — The root seeking moisture displays Hydrotropism. 



587. Contact. — When the revolving end of a tendril or a twining 

 stem strikes an object of support, growth on the touched side is re- 

 tarded. The effect of this stimulus is, therefore, to make the tendril 

 or stem encircle the support. 



588. Variations of light and heat modify the rate of growth on oppo- 

 site sides of leaves. If the upper surface of blade and petiole grows 

 faster than the lower, the whole leaf is depressed ; if the lower side 

 grows faster, the leaf is raised. Movements of this sort are especially 

 noticeable in floral leaves. In warm sunshine, for example, the leaves 

 of the Dandelion head unfold for the visits of insects ; but when, in 

 the afternoon, the light and warmth fall off somewhat, the bracts and 

 corollas of the inflorescence close up tightly. In other cases the effects 

 of illumination are just the reverse, for the flowers open at night, 

 when the nightfliers that pollinate them are abroad. 



589. Movements due to change of turgidity. — These have been 

 described in the chapter on the leaf (sleep movements, behavior of 

 the Sensitive Plant, action of insectivorous leaves). Such movements, 

 due to changes of turgidity (apart from growth), are confined to leaves 

 (vegetative and floral) ; and they result from the sudden escape of 

 water from the swollen tissues of the pulvinus or other motile organ, 

 into the internal ducts or intercellular spaces. 



590. Irritability. — All the movements and changes of movement 

 referred to in §§ 583-589, occasioned by external exciting causes 

 (stimuli), are manifestations of the irritahility \\\\\Qve\\t in protoplasm. 



