392 



VEGETABLE GROWTH. 



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

 wliieh do not exhibit this phenomenon. 



1026. Geotropism. Suppose a j'oung 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 gi'avitation, 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. 



especially for lajing 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 flnallj- results in the assumption of the 

 erect position bj- 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 geo- 

 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 



Fig. 17a Vicia Paba Descent of root into mercury. (Sacbs.) 



