402 STORAGE SYSTEM 



supplied with water, it quickly regains its former dimensions ; the 

 folded radial walls stretch out again under the influence of the 

 turgor-pressure developed as a result of the reabsorption of water. 

 This power of recovery after repeated depletion naturally also consti- 

 tutes one of the important qualifications of an efficient water-tissue. 



The above concise statements may be supplemented by a few 

 explanatory remarks. The depletion of a water-tissue in time of 

 drought involves two quite distinct processes. A comparatively small 

 amount of the accumulated water escapes in the form of vapour, 

 partly through the outer epidermal walls, and partly by way of the 

 ventilating spaces which are present in all water tissues, although 

 their development is there relatively feeble. A far larger proportion 

 of the water removed is absorbed by the photosynthetic system, by 

 virtue of the higher osmotic pressure which prevails in the green cells ; 

 the latter are thus enabled to cover the loss which they suffer by 

 transpiration, at the expense of the water-storing cells, for a very con- 

 siderable period. The protection which the water-tissue affords to the 

 photosynthetic system, when the supply of water is insufficient, accord- 

 ingly serves not only to preserve the latter from fatal injury through 

 desiccation, but also to prevent any temporary interruption of photo- 

 synthetic activity through shortage of water. So long as plenty of 

 water is available, the green parenchyma can obtain all the water and 

 mineral salts that it requires from the vascular bundles ; but if this 

 normal source of supply fails for any reason, the photosynthetic cells 

 can still fall back upon the store accumulated in the water-tissue. 



Contraction of the epidermal water-tissue does not at first entail 

 any folding of the radial cell-walls. To begin with, namely, the dis- 

 tended walls contract elastically, as the pressure in the protoplasts 

 diminishes. If a suitable leaf, or a transverse section thereof, be 

 immersed in a solution of common salt of sufficient strength, the turgor- 

 pressure of the water-tissue cells will be completely neutralised, and 

 the elastic tension in the cell- walls relieved ; by means of this plasmo- 

 lytic method, it is possible to determine, with a fair amount of accuracy, 

 the extent to which the collapse of the water- tissue is referable to the 

 mere elastic contraction of its radial walls. A number of experiments 

 were carried out in this way by the author upon leaves of P&peromia 

 trichocarpa ; in every case, measurement revealed a radial contraction 

 equivalent to nearly 6 per cent, of the total thickness of the water- 

 tissue, when the turgor-pressure had fallen to zero. It is quite clear, 

 therefore, that the mechanism of contraction is not the same for small 

 variations of water-content, as it is for a serious loss of water. 



According to Schwendener, a very unusual type of water-tissue is 

 found in the Makantacea.e, occupying the periphery of the heliotropi- 



