CHAPTER VII. 



THE WATER-RELATION. 



If you neglect to water plants grown in pot culture in the greenhouse 

 the plants will wither : if the withering has not gone too far the plant 

 may recover after watering ; but if the neglect has been too prolonged 

 the plant will die. These facts are familiar to everyone who has 

 experience of indoor cultivation of plants ; and though the problem of 

 water-supply may perhaps be less obvious in plants growing naturally 

 in the open, it is no less a grave one for them also. 



In discussing the relation of plants to water it is necessary in the first 

 place to realise how high is the proportion of water contained in ordi- 

 nary plants. If a block of wood be cut out from the trunk of a living 

 tree, and weighed, and after drying out thoroughly at 100° C, it be 

 weighed again, it would be found to have lost about half its weight. 

 Thus water forms about half of the weight of so solid a tissue as 

 the wood in the normal living state. In succulent tissues of the 

 leaves or young stems, or in the tissues of herbaceous plants, the pro- 

 portion is much larger. In the case of the fresh Cabbage it amounts 

 to about 92 per cent., and in the Lettuce, as cut fresh for a salad, to 

 about 95 per cent. Thus only about 5 per cent, of a crisp Lettuce 

 consists of the substance of protoplasm, and cell-walls. The living 

 plant may then be regarded as a structural framework retaining within 

 it a very high water-content. The water exists there in various forms. 

 A large part of it appears as liquid water, filling the vacuoles of cells, or 

 in the cavities of the vessels, and it can be seen as such microscopically. 

 But a considerable proportion of it is absorbed into the substance of 

 the protoplasm or the cell-walls and starch-grains, as water of im- 

 bibition, upon which their swollen condition depends. Some may be 

 present as " water of constitution," entering more intimately into 

 relation with the substances of which the plant body is composed. 



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