380 LECTURE XVI. 



The Tonic Influence of L ight. Phototonus. The response 

 of a growing organ to the action of a stimulus, by an altera- 

 tion in either the rate or direction of its growth depends, as 

 we have already learned, upon two conditions, namely, that 

 the protoplasm should be irritable and motile, and that the 

 mechanical structure permits of the movement (p. 341). In 

 some cases, organs,.- notably dorsiventral leaves, cease to 

 exhibit irritability, and in fact cease to grow altogether, when 

 they are kept for some days in continuous darkness. On being 

 exposed for some short time to light they regain their irrita- 

 bility, as is clearly shewn by the fact that they then respond 

 by variations in their rate of growth to variations in the inten- 

 sity of the light to which they are exposed. Similarly, ex- 

 posure to light of great intensity induces a loss of irritability 

 and leads to the arrest of growth. Wiesner has found, for 

 example, that the heliotropic ettect of light diminishes when 

 the intensity of the light to which the organ is exposed 

 exceeds a certain optimum which varies with the plant, and 

 disappears altogether at a certain maximum intensity, which 

 may be either somewhat higher or lower than that at which 

 growth is- arrested. The peculiar condition induced by 

 exposure to light of a certain intensity, in which protoplasm 

 is capable of exhibiting irritability, has been called by Sachs 

 Phototonus. 



We may conveniently consider here the general question 

 of the effect of continuous darkness and of subsequent ex- 

 posure to light upon the development of plant-organs. -Oe 

 of the most striking features presented by plants which have 

 beeiTgrown in darkness is the smallness of the leaves. This 

 is not a universal rule, by any means, though it applies in 

 the vast majority of cases when the leaves are dorsiventral. 

 The radial or bilateral leaves characteristic of many Mono- 

 cotyledons become excessively elongated in darkness, just as 

 shoots do, but their breadth is diminished. And even among 

 dorsiventral leaves exceptions occur; the leaves of the Beet, 

 for example, attain a considerable size in darkness. From 

 Sachs' observations it appears that leaves which, when they 

 unfold under normal conditions, become fully exposed to light 



