1-2 IWYI'TIAS AGRICULTURE. 



the sap do not readily pass out through the protoplasmic 

 layer, a certain amount of outward diffusion goes on. 

 Among the substances thus passed out one of the principal 

 is carbonic acid which aids considerably in the solution of 

 the soil particles. 



In the case of salt soils, the soil water is a fairly strong 

 solution, and the inward flow is much reduced. As the 

 land dries, the solution in the soil becomes stronger and 

 stronger, and an outward flow may at length be set up. 

 This is fatal to the life of the plant, and hence the import- 

 ance of plenty of irrigation water in the cultivation of 

 salt lands. 



The water taken in by the plant is drawn not only 

 from the particles in the immediate neighbourhood of the 

 root but also from comparatively distant particles. In 

 explanation of this consider the case of a group of particles 

 touching one another. If this group receive water, this 

 water will distribute itself throughout the pores and over 

 the surfaces of the particles in such a way as to be in 

 equilibrium. This it does under the influence of what is 

 known as surface tension, a property of liquids which 

 cannot be fully entered into here but upon which the 

 behaviour of the surface of a liquid depends. If water be 

 drawn from any particle of the group, this equilibrium is 

 disturbed and there is a flow of water from all the other 

 particles towards the said particle. In this way a root- 

 hair, when it draws water from a particle imbedded in its 

 mucilaginous layer, starts a current of water towards itself 

 from every particle in its neighbourhood, since every 

 particle of soil may be regarded as in communication with 



