374 HOW CROPS GROW. 



tissues for sap. When a fresh stem or leaf loses a few 

 per cent of water, it becomes flabby and, except so far as 

 supported by indurated woody-tissue, has no self-sustaining 

 power and droops from an upright direction. On dissect- 

 ing the flabby stem lengthwise, the halves no longer curve 

 apart, and the tension noticed in the fresh stem does not 

 exist. The water being restored through the root, the 

 normal turgor and original position are both recovered. 

 In the cell-tissue, the cells themselves, so long as tension 

 manifests itself, are fully occupied and distended with sap, 

 and contain a highly osmotic protoplasm ; the vascular 

 tissues being the result of age and alteration in the cell- 

 tissue, are therefore more rigid in their walls and less 

 sensitive to mechanical strain. 



Upward Growth, If a stem whose terminal parts are 

 in a state of highly unequal tension be brought into a 

 horizontal position, it will be found that as it makes new 

 growth the tip curves upward until it becomes vertical. 

 This is due to the fact that while the whole growing part 

 elongates, the under side extends most rapidly. Hof- 

 meister has demonstrated that this curvature is not the 

 result of increased tension in the active cell-tissue of the 

 lower longitudinal section of the stem, but of increased 

 extensibility on the part of the cuticular and vascular tis- 

 sues of that region, for on removing the entire cuticle 

 from a curved onion-stalk the curvature was not increased 

 but diminished. 



The question now arises, why do the passive parts of 

 the under side of the stem that is out of the vertical ad- 

 mit of greater expansion by the stress of the rapidly 

 growing tissues, than those of the upper? The only 

 cause hitherto assigned is the action of gravitation on the 

 juices of the tissues. In a stem inclined from the verti- 

 cal, the cells of the lower side experience not only the 

 general pressure of the water which renders the whole 

 turgid, but, in addition, they sustain a portion of the 



