250 PART III. — VEGETABLE PHYSIOLOGY. 



ference, that the stem grew inward toward the centre of the 

 wheel, or in a direction opposed to the pull, while the root grew 

 outward from the centre, or in the same direction as the pull, 

 indicating that in the ordinary growth of plants, gravity is the 

 cause of the downward trend of the root, and also that of the upward 

 trend of the stem. This conclusion is confirmed by other experi- 

 ments. The condition of the growing tissues of the root is such 

 that when that organ is stimulated by a constant downward pull 

 it grows downward, while the different condition of the tissues of 

 the stem causes that organ to grow in the opposite direction 

 under the influence of the same stimulus. Under ordinary condi- 

 tions, other forces, as we shall presently see, more or less 

 modify the action of gravity; still it is mainly this which deter- 

 mines the position of the various organs of the plant with respect 

 to the horizon. It is not only the chief cause of the downward 

 growth of the main root of a tree, and of the upward growth of 

 the stem, but it has much to do with the horizontal or oblique 

 growth of the branches and leaves, and if any young and growing 

 organ be by any cause diverted from its wonted direction, it 

 tends to resume its normal direction again when the disturbing 

 cause is removed. An organ which grows directly downward or 

 in the direction of the pull of gravity is said to be positively geo- 

 tropic ; one which grows directly upward, or in opposition to 

 that pull, is said to be negatively geotropic or apogeotropic, and 

 one which assumes a position at right angles, or nearly so, to 

 the pull is said to be transversely geotropic. 



(3) Heliotropism. This term is applied to movements caused 

 by the stimulant effects of light. The effect may be either to 

 cause the organ to curve toward the source of light, or to bend 

 away from it. In the former case the plant or organ is said to 

 be positively heliotropic, and in the latter, negatively heliotropic 

 or apheliotropic. Plants growing in the open sunshine of course 

 always have one side more strongly illuminated than the other, 

 but owing to the diurnal motion of the earth, the effect of which 

 is to cause the direction of the light to constantly change, and 

 owing also to the slowness with which most organs respond to 

 the stimulus, movements of this character are not ordinarily 

 conspicuous. It would not be difficult to prove, however, that 

 the position which leaves and some other organs assume is due 



