674 THE POPULAR SCIENCE MONTHLY. 



HELIOTROPISM : THE TURNING MOTIONS OF PLANTS. 



By CONWAY McMILLAN. 



AS its derivation would indicate, heliotropism means " turning 

 toward the sun/' and is the technical name applied to all 

 such phenomena in the vegetable kingdom. It was well known 

 to the ancients that plants exhibited a remarkable sensitiveness to 

 light, for Aristotle mentions it, and indeed, in its more apparent 

 forms, it is conspicuous even to the naive observer of to-day. The 

 sunflower, or tournesol, as the French name it, follows the daily 

 course of the sun with its disk-like inflorescence ; plants, potted 

 and placed in a window, bend toward the light, unless, perchance, 

 the plant is an ivy, in which case it bends away from the source 

 of illumination ; trees and shrubs in the edge of a thicket or for- 

 est may be seen to slant toward the open, and in general it may 

 be said that there is scarely a plant which does not respond more 

 or less distinctly to the directive action of light. Exceptions, as 

 shown by Darwin and by Edouard Morren in his treatise on in- 

 sectivorous plants, are for the most part carnivorous species like 

 Dionea, Drosera, and Nepenthes — the Venus's-flytrap, sundew, and 

 pitcher-plant, respectively — and twining plants. The reason why 

 these plants should not fall under the rule will be apparent when 

 the uses of heliotropism are discussed. Parasitic and the so-called 

 saprophytic plants of the lower orders — those which live upon 

 once-living matter — are commonly insensible to heliotropic stimu- 

 lus, and, in short, all plants devoid of the great light-product — 

 chlorophyl — manifest in this direction either weak irritability or 

 none at all. 



Heliotropism, it must be remembered, is not confined to plants 

 as individuals, but is manifested by the different organs in vary- 

 ing degrees. Tendrils, for example, are either distinctly helio- 

 tropic, or far more commonly apheliotropic, as Darwin calls it — 

 that is, negatively heliotropic ; leaves are transversely or diahelio- 

 tropic — in other words, they tend to place themselves perpendicu- 

 larly to the incident rays ; stems, flower-peduncles, even roots, each 

 in its own way, reply to the stimulus of lateral light. In passing, 

 it should be mentioned that a plant has but one way of responding 

 to conditions without, and this is by curvature. Even the sleep 

 of leaves, the spontaneous movements of the sensitive-plant, or of 

 that singular pulse, the Hedysarum gyrans, in which the two 

 lateral leaflets keep up an incessant jerking motion ; the reaction 

 to a cut, bruise, or wound of any kind — as may be seen in an in- 

 jured tendril ; the effect of ether or chloroform, and indeed of natu- 

 ral forces such as electricity, gravity, or heat-vibrations, is in every 



