606 PHYSIOLOGY. 



normal circumstances, they would have begun to feel the effect of the 

 light that is, at the moment of their emergence from the bud. When 

 the leaf has to shift for itself, starch is formed in those cells exposed to 

 the light first, and all subsequent growth depends upon the formation of 

 that starch. In darkness no starch is formed, and hence, according to 

 Kraus, the arrested development of the growth of the leaf. But Rau- 

 wenhoff has shown that the young leaves cannot get all their nutriment 

 for themselves, but must derive some from the reserve stores ; if. by 

 means of incisions of the petiole, this be prevented, the growth of the 

 leaf is arrested ; and he concludes that the explanation of the arrest of 

 growth in leaves in obscurity cannot be yet given. 



The lengthening of the stem in darkness, according to the same observer, 

 depends upon the differences of tension between the active pith-cells 

 and those of the fundamental tissue generally and the passive 

 cells of the wood and bark. In normal stems the pith has always 

 a tendency to elongate, a tendency checked by the resistance of the 

 peripheral layers. In darkness, on the other hand, as ascertained also by 

 Rauwenhoff, the bark-cells and wood -cells either are not formed or 

 do not thicken, and hence do not offer any resistance to the growth of 

 the pith-cells. 



Reinke has drawn attention to the fact that in previous experiments 

 on plants grown in obscurity sufficient attention has not been paid to the 

 concomitant effect of moisture. He therefore first studied the influence 

 of moisture on the growth of Helianthus annuus, taking two seedlings and 

 placing one in the open air, the other under a receiver in fully saturated 

 air. It was found that in the saturated air the growth was quicker than 

 in the ordinary air, yet it fell short of the acceleration which etiolated 

 plants (growing in the dark) had shown. Under the glass vessel in light 

 the stems seemed merely to reach their normal size sooner than in dry 

 air ; whereas the darkening caused a quite anomalous extension of the 

 length. This darkening and greater moisture have both an accelerating 

 action on the intensity of growth. 



Heliotropism. Plants exposed to the light bend towards the quarter 

 whence it comes, and become more or less concave towards that side. 

 The side furthest from the light, or, in other words, the darkest side, 

 grows faster than the illuminated side, in accordance with what has been 

 just said. The illuminated side thus resists the growth of the other side, 

 and a curvature is formed, the concavity towards the light, the convexity 

 in the opposite direction. This curvature is called heliotropism, and is 

 manifested by plants which have no chlorophyll, as well as by those which 

 possess that substance. Rays of high refrangibility, such as blue or violet, 

 cause heliotropism by retardation of growth. Heliotropism of the kind 

 mentioned is called positive to distinguish it from negative heliotropism, in 

 which the exact opposite occurs, the illuminated stems becoming convex, 

 as also in the case of root-hairs, some tendrils, the roots of CMorophytum, 

 &c. The kind and degree of curvature are shown by Wolkoff to vary 

 according to the part of the root affected. The same observer also shows 

 that in some instances of negatively heliotropic roots, the translucency of 

 the tissues is such that the rays of light may be refracted, so as to pro- 

 duce a more intense illumination on the side furthest from the light, the 

 concavity of which surface would therefore be a true instance of positive 



