2-tO BRIEF OUTLiyE OF VEGETABLE PUYSIOLOGY 



those of tentacles, some staiiiens, and leaves of tlie Sensitive Plant are 

 due to alterations in tissue tensions independent of growth. 



581. !lIovenie)its nniy lie due : (1) lo internal causes, or ('2) to ex- 

 ternal influences. 'J'he tii'st are spoiiianedus, the second induced. 



582. Spontaneous growth movements. — Darwin showed that the 

 tips of growing plart.^ of plants — stenr<, leaves, roots — perpetual! v 

 move in irregular eiliptieal curves. Since the^motion is one of Low- 

 ing toward all points ot the compass in turn, he called it circiuiinulntini,. 



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

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

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



584. Gravity. — It lias heeu observed from actual experiment in 

 the laboratory that I'oots of .seedlings turn toward the center of the 

 earth, while the pkinude turns toward the zenith. All turnings undnr 

 influence of gravitative force are manifestations of (Jeclmpixm. Ihe 

 root is said to lie p'ositively, the shoot negatively, r/enlrnpic. 



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

 If one could oliserve the i oot, ho\vever, it would tie found to turn awav 

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

 is, in general, positively hcliolropic, the root negatively helintropic. 



586. Moisture. — The : oot seeking moisture displays Hi/ilniim/jisin. 



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

 stem strikes an oliject of snppiort, growth on the touched side is re- 

 tarded. The eft'ect of this stimulus is, therefore, to make the tendril 

 or stem encircle the sn]"Tiort. 



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

 site sides of leaves. If I lie nppier 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. Jlovements of this sort are especially 

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

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

 the afternoon, the light and waianth 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, 

 v,']vu the nightliiers that pollinate them are abroad. 



589. Movements due to change of turgidity. — These have bet-n 

 desci-ibed in the cliapter on the leaf (sleep movements, behavior nl' 

 the .Sensitive Plant, action of insectivorous leavi-'s). Sncli movement-, 

 due to clnrn ges of turgi':lit\- (apart f]x»iii gnuNJ h ), are confined to leaves 

 (vegetativf and floral) ; and they result fi-mii the sudden escape of 

 water from t!ie swollen tissues of the pmlvii ns or other motile organ, 

 into ihe iiili-rnal ducts ur intercellular spaces. 



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

 referred t.. in ^< .583-589. ocra^ioued by cxbuaial exriting cau.-es 

 (stimuli), are manifestations of the irrtUihilt/// inherent iu pr(jtojila.-,ni. 



