392 VEGETABLE GROWTH. 



some parts, as for example most roots near their extremity, 

 which do not exhibit this phenomenon. 



1026. Geotropism. Suppose a young shoot to possess the ten- 

 sion already described ; let this be placed, while growing, in an 

 horizontal position. In consequence of its position the nutri- 

 ent fluids will, from the force of gravitation, have a tendency 

 to collect in greater amount in the cells upon its under side. 

 Their presence on that side will not only cause an increase 

 of turgescence there, but will offer to the growing cells a larger 

 amount of available material for immediate use in growth. 



especiall}- for laying down the cell-wall. From one or from 

 both of these causes there will therefore be an appreciable elon- 

 gation of the tissues on the under side, and hence a curving up- 

 wards will occur, which finally results in the assumption of the 

 erect position by the organ in question. 



1027. If, on the other hand, the organ possesses little or no 

 tension, it is conceivable that the growth would result in a cur- 

 vature of the extremity towards the ground ; this is seen in the 

 case of roots. The same factors produce an upward curvature 

 where there is marked tension of tissues, and permit a down- 

 ward curvature where there is little or no tension. It is a sig- 

 nificant fact that in the case of certain branches from roots the 

 direction of growth is oblique. 



1028. Organs which turn towards the earth are termed ffeo- 

 tropic; those which turn upwards are apogeotropic / those which 

 pursue in their growth oblique directions have been termed 

 diageotropic. 



1029. Heliotropism. It can be shown by exact measurement 

 that in many cases light, especially the more refrangible part of 



FiO. 173. ViciaFaba. Descent of root into mercury. (Sachs.) 



