CHAPTER VI. 



THE WATER-RELATION. 



If you neglect to water plants grown in pots, 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 grown plants in pots ; 

 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 w^ood 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 89 p.c, and in the Lettuce, as cut fresh for a salad, to about 

 96 p.c. Thus only about 4 p.c. 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 protoplasts, 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 constittdion,'' entering more intimately into 

 relation with^the substances of which the parts are composed. 



