INFLUENCE OF GRAVITATION ON GROWTH. 76 1 



If we now apply the results of my experiments on internodes and nodes of 

 Grasses which curve upwards to the simple tube, the growth is found to be more 

 rapid on the convex under side, less rapid on the upper side of the upwardly curved 

 part, than when it grew erect. It may be assumed, from Ciesielski's measurements 

 of roots, that when the tube curves downw^ards the growth is more rapid on the 

 convex upper side, less rapid on the concave under side, than when the curved part 

 grew for a longer time in a vertical direction. In other words, when the tube is 

 placed in a horizontal position the growth is accelerated on the upper side of the 

 positively geotropic part and on the under side of the negatively geotropic part, but 

 always retarded on the opposite sides. 



If therefore we assume that in Fig. 452 ^ the two side walls of a transverse disc 

 of the part 6" of the tube when in an upright position had lengthened in a definite 

 time to the equal lengths and u u, it would have remained upright ; and if the 

 tube had been placed horizontal during this time, the lower side would have attained 

 the greater length 11 ii, the upper side the shorter length 0' o\ and the piece must in 

 consequence become curved. Exactly the opposite would be observed, as shown 

 in Fig. 452 C, if the growing piece belonged to the part fFof the tube. 



If now the unicellular tube A were supposed divided by transverse and longi- 

 tudinal divisions into a tissue consisting of a number of layers of cells ; or if, what 

 amounts to the same thing, a stem of a seedling were supposed substituted for the 

 part 6" of the tube, and a root for the part W, the same phenomena w^ould occur, 

 as experiments have shown, in every cell of the growing part, as those previously 

 observed in the tube. In the part S every cell would grow^ more rapidly on the under 

 side, less rapidly on the upper side than if the part were upright, the reverse in the 

 part W. We should find that in -S" both the upper and under sides of any cell 

 {i. e. upper and under in relation to the radius of the earth) are longer than those 

 of the cells situated above it, the reverse in W; in other words, that every individual 

 cell of a part which shows geotropic curvature behaves in the same way as if the 

 part previously straight were held firmly by the two ends and then bent. This 

 will be made clearer to the student if in the portion of the curved part included in 

 A lines are drawn parallel both to the straight and the curved outlines, and the septa 

 of the cells are then indicated in the straight piece simply by parallel lines crossing 

 the first at right angles, in the curved part by lines corresponding to the radii of 

 curvature. The cells exposed by longitudinal sections through nodes of Grasses 

 and roots endowed with geotropic curvature exhibit this phenomenon, although 

 with many irregularities. 



When the facts connected with the geotropism of the cell-wall have thus been 

 made clear, we may proceed to the question, w^hy or by what effect of gravitation 

 these differences are occasioned in the growth on the upper and under sides of 

 every cell of a geotropic organ when placed in a horizontal position. We have 

 at present however no answer to this question, any more than in the case of helio- 

 tropism, the same diagram availing, mutatis mutandis^ for the two phenomena. 



The view brought forward by Hofmeister, and for some time adopted by me, 

 that positive geotropism occurs only in those organs and in those parts of organs 

 in which there is no tension in the tissues, while the organs in which there is strong 

 tension are negatively geotropic, rested on imperfect induction. On the one hand 



