SECT. II 



PHYSIOLOGY 307 



proceeds from the lamina. Still more striking relations are met with 

 in the seedlings of certain Grasses ; in some Paniceae only the tip of 

 the so-called cotyledon can be phototropically stimnlated, and only 

 the hypocotyledonary segment of the stem, separated by some distance 

 from the tip of the cotyledon, is capable of curvature. In this case 

 there is a well-marked distinction between a perceptive organ and a 

 motile organ ; the similarity to corresponding phenomena in the 

 animal kingdom is very striking. There is an essential difference, 

 however, in the method of transmission of the stimulus. For not 

 only are "nerves" completely wanting in the plant, but the stimulus 

 is conveyed from, cell to cell ; as Fitting showed, this does not only 

 take place in a direct line, but also " round the corner." 



There i.s no doubt that the perception of light bj' the plant is closely connected 

 with photochemical processes C*^^). As to how the plant perceives the direction 

 of the light we are, however, ignorant. Haberlandt attempted to show that the 

 structure of the epidermal cells brought about an unequal illumination of their 

 inner walls. When the rays of light fell vertically, the middle of the wall would 

 be most brightly illuminated. When any other distribution of light was brought 

 about the plant made a phototropic movement. The projection outwards of the 

 outer walls of these cells (p. 112), or thickenings in these walls (p. 76), act as 

 convex lenses, and increase the difference in the illumination between the centre 

 and periphery of the inner walls of the cells. This hypothesis is still open to 

 considerable doubt. 



(b) The Stimulus of Gravity ( "») 



Geotropism. — It is a matter of experience that the stems of a 

 fir-wood or of a palm avenue are all vertical, and therefore parallel to 

 one another ; the branches and leaves of those trees, on the other 

 hand, take other positions. If, instead of a tree, we consider a 

 seedling, for example of the Maize, we find that, at any rate to begin 

 with, the organs stand in the vertical line. At the same time we here 

 observe more readily than in the case of a tree the totally diflTerent 

 behaviour of the roots and the stem. If we bring the seedling from 

 its natural position and lay it horizontally we find that a curvature 

 takes place in both organs ; the root curves downwards, and the shoot 

 of the seedling upwards. Since these curvatures are not eftected at 

 the region where the root passes into the shoot, but in the neighbour- 

 hood of the apices of the two organs, a region of variable length 

 remains horizontal, and only the two ends of the plant are brought 

 by the curvature back into their natural directions, and continue to 

 grow in them. That this vertical growth of the main root and main 

 stem is ilue to gravity is apparent from direct observation, which 

 shows that these organs are similarly oriented all over the globe, and 

 lie in the direction of radii of the earth. The only force acting 

 everywhere in the direction of the earth's radius that we know of is 

 gravity. Not, however, as a result of this line of thought, but from 



