GEOTROPISM. I 429 



LECTURE XXXIV 

 GEOTROPISM. I 



No special botanical knowledge is required to convince oneself of the fact, 

 which all practical experience teaches us, that plant organs assume certain 

 definite positions in space. The tree trunks in a fir-wood all stand perfectly 

 erect and are hence all parallel to each other ; their branches, large and small, 

 always conform to rule, still their lie cannot be stated only in terms of the 

 angle which they make with the perpendicular, since that obviously does 

 not comprise all the relations they bear to the chief axis for the time being. 

 Instead of a fir-tree let us examine a seedling, thus simplifying the problem, 

 since, in this latter case, the only organs present, at least at first, are those which 

 grow perpendicularly. At the same time we can observe here, much more 

 readily than in the case of a tree, the totally different behaviour of the root and 

 the stem. Both grow perpendicularly to the earth's surface, but the stem 

 grows upwards, while the root grows downwards. If we place the seedling 

 in an unnatural position, e.g. horizontally, we note immediately that both 

 organs begin to bend, the root downwards, the plumule upwards. Since 

 these curvatures take place, not at the place where stem and root meet, but 

 near the apices of both organs, a varying length of axis remains horizontal, 

 and only the two terminations resume the perpendicular position on bending, 

 growth being continued in that direction. Since almost every organ in the 

 plant has a certain definite position of rest and endeavours to regain it after it 

 has been interfered with, we must grant to the plant the capacity of orientating 

 itself in space, and the movements which its members exhibit in their endeavours 

 to assume their natural and appropriate positions, not by simple bending merely 

 but also by torsions and twinings, we term movements of orientation. Obviously 

 this orientation is the result of the action of certain external factors such as the 

 distribution of light, water, &c., and the plant must possess sense organs of 

 some kind by means of which it appreciates the influences thus brought to bear 

 upon it by the environment. 



Very frequently the orientation of an organ is dependent on the combined 

 influence of several factors, but in the simple cases with which we wiU begin, as 

 the downward curving of the root and the upward curving of the shoot, ap- 

 parently one agent only is concerned, viz. gravity. That gravity is directly re- 

 sponsible for the perpendicular mode of growth of root and shoot may be shown 

 by direct observation, for these organs are orientated in the same way over the 

 whole surface of the globe, that is, parallel to the earth's radii, and we know of no 

 other force which acts universally in this direction. Still it is not on reflections 

 such as these but on the experiments of Knight (1806), and Sachs (1874) that 

 our knowledge of the subject is actually founded. Knight's experiments rest 

 on the following basis : — Obviously gravity can cause the root to grow down 

 and the shoot to grow up only if the seed remain in a state of rest and in the 

 same relative position with reference to the direction of the earth's attraction ; 

 hence Knight concluded that if the lie of the germinating seed were continuously 

 and rapidly changed by being subjected to the influence of another, say centri- 

 fugal force, the efifect of gravity might be suspended. 



He, therefore, fastened a number of germinating seeds to the rim of a wheel 

 in a variety of positions, so that the protruding radicles pointed outwards, 

 inwards, and tangentially, and rotated the wheel on a horizontal axis. As the 

 wheel was made to revolve at a very considerable speed, not only was the 

 unilateral influence of gravity neutralized, but at the same time a very consider- 



