842 MECHANICS OF GROWTH. 



like W\ the part ^S" on the contrary the upward curvature, as S'. It is self-evident 

 that each of these curvatures can only result from the growth, equal on all sides 

 when the organ is erect, having now become unequal on the upper and under sides, 

 the convex growing in both cases more quickly than the concave side. 



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 filament curves downwards the growth has been more rapid on 

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

 had grown onwards in a vertical direction. In other words, when the filament 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. 482 ^ the two side walls of a transverse disc 

 of the part -S of the filament when in an upright position had lengthened in a definite 

 lime to the equal lengths and u u, it would have remained straight ; but if the 



Fig. 482.— Diagram for illustratinji geotropic upward and downward curvature. 



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

 the greater length u u, 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. 482 C, if the growing piece belonged to the part PFof the filament. 



If now the unicellular filament A were supposed divided by transverse and lon- 

 gitudinal 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 to be substituted for 

 the part 6* of the filament, and a root for the part PF, the same phenomena would 

 occur, as experiments have shown, in every cell of the growing part, as those pre- 

 viously observed in the filament. 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 indi- 

 vidual 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 



