GROWTH, IRRITABILITY AND MOVEMENT 157 



they are often present in the starch-sheath in the younger part of the stem. 

 When a root or a "stem is displaced from its normal position and placed, say 

 horizontally, the statoliths under the influence of gravity move until they 

 lie against the wall which is now lowermost (Fig. 95, B, C). They will then 

 be pressing on a part of the cytoplasm that is not customary : the theory 

 suggests that this unusual pressure activates some mechanism which results 

 in growth being altered in such a way as to produce a curvature. This curva- 

 ture ultimately results in the statoliths returning to their normal position. 



Though the statolith theory of gravity-perception is not universally 

 accepted, there is considerable evidence in its favour. Experiment shows 

 that in roots the sensitiveness to gravity is largely confined to the tip, i.e. to 

 the region where the statoliths are present. The remaining part of the root, 

 even the zone of cell-elongation where the geotropic curvatures are effected, 

 is relatively insensitive. That this is so is suggested by the observation 

 that a root which has been decapitated can be placed horizontally without 

 any resulting curvature, although the zone in which curvature would be 

 expected has not been removed. It might be suggested that this was merely 

 due to the injury involved in decapitation, but the fact that the ability to 

 execute curvatures is restored if the tip is replaced in position by means 

 of a drop of water or a thin layer of gelatin is against this view. In the 

 coleoptile of cereals the position is similar to that in the root : the statoliths 

 occur in the apex and sensitiveness to gravity is chiefly resident there. In 

 stems the apical part is in general again the most sensitive, though there is 

 not the same degree of localisation of sensitiveness as in the root. This is 

 in agreement with the occurrence of the statoliths, which are distributed 

 through the younger part of the stem. 



Much attention has recently been paid to the importance of hormones 

 in the production of geotropic curvatures. Evidence has been obtained 

 that the development of a curvature follows on a re-distribution of the 

 growth -controlling hormone, which, as we have seen (p. 150), diffuses back 

 from the apices of coleoptiles, of shoots and probably of roots, to the zones 

 of cell-elongation. Under normal conditions the hormone arrives at the 

 zone of elongation equally distributed all round the organ : when the 

 organ is placed in an unusual position with respect to gravity the movement 

 of hormone appears to be in some way so affected that more arrives on the 

 lower than on the upper side of the growing zone. Since the hormone is 

 growth-promoting in the stem and coleoptile, and growth-retarding in the 

 root, an upward curvature is the result in the former, a downward in the 

 latter. According to this explanation the importance of the apex in geo- 

 tropic curvatures is that it is the source of the growth-controlling hormone. 

 The significance of the statoliths in these events is uncertain. 



It has been remarked that geotropic curvatures in general are 

 only shown by young still-growing parts of plant organs, the mature 

 parts being relatively incapable of curving. An exception to this 

 statement is provided by the stem (haulm) of cereal Grasses, in 

 the lower part of which curvatures may be developed. The nodes 

 of these shoots, which are really the swollen bases of leaf-sheaths, 



