9 6 BOTANY OF THE LIVING PLANT 



The significance of water to the plant is manifold. It is the medium 

 in which the protoplasm conducts its chemical reactions and it is 

 actually used up in a number of chemical operations, e.g. in photo- 

 synthesis ; it is the solvent through which all materials enter the 

 plant cell from its environment, and thus they pass from point to 

 point in the plant : finally, its accumulation at an early stage under 

 pressure within the cells leads to their enlargement, and subsequently 

 gives firmness to them and to the organs that they compose. 



It is clear that an active plant will require constant supplies of 

 water for its chemical operations, and to meet the needs of new tissues. 

 But over and above the small amount of water required for these 

 purposes, a very much greater quantity is needed to make good the 

 loss incurred by the evaporation of water (or Transpiration) from the 

 aerial organs. 



Absorption of Water by the Plant. 



Land-plants rely on the soil for their water supply. But the 

 absorption of water is mainly restricted to the younger parts of the 

 root system, and especially to the regions bearing root-hairs. The 

 structure and distribution of these hairs have been described in the 

 previous chapter, where it was pointed out that their presence increases 

 the absorptive area of the root very considerably, while the hairs 

 make intimate contact with particles of the soil (Fig. 60), facilitating 

 absorption of water present as films round those particles. Each 

 root-hair-cell forms an osmotic system, as described for plant-cells 

 in general in Chapter III. ; the cell-sap normally has an osmotic 

 pressure of 5 atmospheres or more, so that the hairs are in a position 

 to carry out osmotic absorption of water under suitable conditions. 



The soil in which the root system develops is a complex mixture 

 of materials, organic and inorganic, holding within it water to a greater 

 or lesser degree. No soil in the open is ever actually dry, and normally 

 the amount of water contained is considerable. Soil-water is of 

 different types. Thus we may recognise as gravitational water that 

 which is only present in badly-drained soils, or after rain. Of the 

 water retained by a well-drained soil, some is held by capillarity in 

 the minute channels between the particles, or in the form of films 

 round the particles ; while some is held by imbibition in the colloidal 

 constituents of the soil, such as clay and humus. Humus, using the 

 term in a broad sense, is the decaying organic matter of the soil, 

 derived from previous generations of plants. Leaf-mould is a type 



