44 BOTANY 



reference should be made to a text-book of plant 

 physiology. 



The water, which is so important to the plant because 

 it holds the necessary food-minerals in solution, is also 

 essential in another way. The living protoplasm must 

 not only be permeated by water, it must have sufficient 

 in it to keep the cells firm and taut. A plant immedi- 

 ately " droops " when the water contents of the cells 

 is reduced, and instead of the stems being brisk and 

 upright and the leaves spread out to the light, the 

 stems and petioles fall and the leaves crumple up. So 

 that, in addition to the food content of the water, the 

 plants need the water itself, and may be as truly said 

 to drink it as that we do so. Many of the organs and 

 tissues of the plant have their part to play in keep- 

 ing the water current going. By the chemical process 

 of osmosis the soft-walled root hairs draw in the water 

 from the soil ; from these cells it passes from cell to 

 cell of the root till it reaches the long, specialised wood 

 vessels (which we noted as of so much anatomical 

 interest), and up these it passes into the corresponding 

 cells and vessels in the wood of the stem, thence, by 

 similar cells in smaller bundles in the leaf stalks, it 

 passes out to the expanded lamina of the leaf itself. 

 There, in the cells of the leaf laboratory, it is chiefly 

 of use, but as each single drop of water contains only 

 a minute amount of nitrates, &c., in solution, any 

 given water drop is soon exhausted, and must then be 

 replaced. Before it can be replaced, however, it must be 

 got rid of, for the cells are each bounded by cell walls, 

 and have, therefore, a limited capacity. The walls of 

 these cells are delicate and permeable, and they are 

 loosely packed in the tissue of the leaf, so that there 

 are many air spaces between them, and this air is in 



