38 PRINCIPLES OF PLANT CULTURE 



Of all the materials obtained by plants from the soil, 

 but three, aside from water, viz., nitrogen, phosphorus 

 and potassium (253) are needed in such quantities that 

 the plants are likely to exhaust the supply, so long as 

 water is not deficient. 



62. Water is necessary to growth. — An adequate 

 supply of water is the most important condition for the 

 well-being of plants, since it not only serves in nutrition, 

 but is the vehicle by which all other food constituents 

 are distributed throughout the plant. Comparatively few 

 soils are so poor as to be incapable of producing good 

 crops when sufficiently supplied with water, while the 

 richest soils are unproductive when inadequately supplied 

 with it. Much of the benefit of manuring undoubtedly 

 comes from the increased capacity it gives the soil for 

 holding and transmitting water (92). 



The supplying of food material is not the only office 

 performed by water in the plant. The unfolding and 

 expansion of the plantlet is largely due to a strong ab- 

 sorptive power for water possessed by the protoplasm 

 within the cells. This force causes all living parts of 

 plants to be constantly saturated with water. Indeed, 

 it distends the elastic cell-walls with water until they 

 are like minute inflated bladders. The pressure thus 

 set up aids in unfolding the different parts from their 

 snug resting-place within the seed-case and enables the 

 plantlet to stand erect. Growth by cell division, it is 

 true, begins rather early in the germination process, 

 but this cannot take place unless the cells are first dis- 

 tended with water (29). A sufficient amount of water 

 is absolutely necessary, therefore, to growth in plants. 

 Foliage wilts in dry weather because the roots are un- 

 able to supply enough water to properly distend the 



