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 
