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PART III. VEGETABLE PHYSIOLOGY. 



The phenomena of heliotropism are similar, in all essential 

 respects, to those of geotropism, except that the force which 

 causes the movement is light instead of gravity. In the one case 

 it is the direction of a pull, in the other the direction of an ether 

 vibration, which, acting upon the irritable living matter of the 

 cell, brings about changes that cause the organ to place itself in 

 a different position. 



It has been found also that movements are sometimes 

 produced by the ultra-red or dark heat rays of the spectrum, 

 some organs moving toward the source of heat, and others away 

 from it. The phenomenon is called thermotropism. 



(4) Hydrotropism is a term applied to organs which, like 

 young roots, have been found to curve toward a moist surface. 

 It is a source of great advantage to a plant, since, by means of 

 it, its roots are, so to speak, able to seek out the moister and 

 avoid the dryer and ordinarily less nutritive portions of the soil. 



(5) Circumnutation. This term was first applied by Darwin 

 to the revolving movements observed in the tips of the young 

 and growing shoots, roots and leaves of the higher plants. The 

 movement consists in a bowing of the organ successively to all 

 points of the compass, thus causing its tip to describe a figure 

 approximating a circle, or, more commonly, an ellipse. It is 

 caused by the formation, lengthwise, of the organ of a line of 

 growth which travels laterally around the organ. 



The circumnutating movements of the growing radicle doubt- 

 less aid it in penetrating the soil ; those of the upper internodes 

 of twining plants are the means by which they climb ; the corres- 

 ponding movements of some other climbers constitute an effi- 

 cient means by which they are able to bring their climbing 

 organs — rootlets, sensitive petioles or tendrils — into contact 

 with a support, and so secure a hold by which they may raise 

 themselves toward the sunlight; and the tendrils themselves are 

 also commonly endowed with circumnutating movements which 

 serve the same useful purpose. This is the most important of 

 all the plant movements. It is also regarded by Darwin as the 

 fundamental form, of which the others are modifications. The 

 phenomena of geotropism, heliotropism, etc., are caused by the 

 modification of this primitive form by external stimuli of various 

 kinds, as gravitation, light, heat, etc. 



