362 PLAN r PHYSIOLOGY 



point is wanned, the free-swimming forms will move towards the 

 warm Bide. As the heat gets greater and greater, a time comes 

 when the movement becomes negatively thermotactic and the 

 organisms swim towards the cooler side oi the vessel. 



Loeb has explained these phenomena on the basis of symmetry. 

 In thermotaxy, for example, as the vessel is warming up, an 

 organism which is bilaterally symmetrica] is warmed more on one 

 side than the other. Reactions on both sides tend to go on at an 

 equal rate, with the result that the organism turns itself in such 

 a manner that both sides may warm up at the same rate, [f it is 

 swimming and headed toward the source of heat, its movement is 

 destined to guide it directly toward the heat, since only when going 

 either directly towards or directly away will both sides be receiv- 

 ing the same set of stimuli. 'This may explain many of the phenom- 

 ena observed, but does not adequately explain the reversal from 

 a positive to a negative response. This explanation has done 

 much, however, to explain instincts on a purely mechanistic 

 basis and to show why some instincts, e, g., the Right of the moth 

 to the flame, may be harmful rather than purposeful. 



Curvature Movements; Imbibition. The ourvature movements 

 o\ plants are duo either to imbibition, to changes in turgor, or to 

 actual growth in the usual sense. If imbibition and increase in 



turgor pressure are also growth (Chap. XXVI), then it can be 



said that all curvature movements are due to some phase of 

 growth. If an organ bends, one side must get longer than the 

 other whether the increase is due to imbibition, osmosis, or increase 

 in the number o( cells. 



The movements which are due to imbibition are hygroscopic 



movements which are purely physical in character but which, 



in some eases, must be o( much importance to the plant. In 



Chapter XIX it was shown that in imbibition water forces its way 



between the particles of material which make up the cell walls 

 and drives them apart, increasing the size oi the imbibing body. 

 Similarly when water is lost, the particles come closer together 

 and the body shrinks. If the one side loses or gains water more 

 rapidly than the other, curvature will result. 



The fruit oi Brodiwn has a long slender beak, which extends 

 straight out from the seed when moistened. When it loses water, 

 one side o\' this beak dries out much faster than the Other. Torsion 

 results and the beak forms a spiral eoil which helps to bury the 



