WATER-ABSORPTION AND TRANSPIRATION. 125 



allows the cell-sap to be forced into the cells of the cortex at the base 

 of the root-hair. Each cell in turn, as we pass inwards in the cortex, 

 repeats this process, alternately becoming turgid and collapsing, so 

 that the water with dissolved salts eventually reaches the central 

 cylinder, and is forced into the wood- vessels. 



The effect of the millions of root- hairs absorbing water and passing it 

 on to the cortex-cells, which pump it into the vessels, is to produce con- 

 siderable pressure, which we may regard as a force driving the water 

 into the wood- vessels and upwards. In some plants it is very marked, 

 especially in spring ; but it can be demonstrated in most plants while 

 active growth is going on and there is sufficient water in the soil. 

 Many plants "bleed" when cut; this is often seen in trimming or 

 pruning plants, especially in spring or early summer. 



159. "Negative Pressure" in Stems. The importance of 

 root-pressure in connexion with the maintenance of the transpiration- 

 current has been greatly over-estimated. It is easy to show that in an 

 actively transpiring plant there is often no root-pressure at all, but a 

 partial vacuum or "negative-pressure." 



Take a Bean seedling which has been pulled up and allowed to 

 lie for some time, till the leaves have begun to wither. Place the 

 seedling with its stem under water coloured with red ink, then cut 

 across the stem under the coloured water. Take the stem out and 

 notice that the water has run up, in the bundles, for some distance. 

 This shows that when the leaves are transpiring and the roots are not 

 absorbing enough water, there is a " negative pressure " in the water- 

 carrying channels, so that when the stem is cut under the coloured 

 water the liquid rushes in. 



16O. Available Plant-Food in Soil. We have seen that 

 root-hairs can only absorb dissolved substances. Salts inso- 

 luble in pure water and required by plants are brought into 

 solution in various ways, chiefly by the carbon dioxide present 

 in soil- water and by the changes due to the various chemical 

 processes we have just mentioned. It is easy to prove that 

 root-hairs give out acid sap, and that they give out carbon 

 dioxide, which accounts partly, at any rate, for the acid 

 reaction they give with litmus. In these different ways the 

 elements essential for plant-food are made available for the 

 use of plants. 



A complete chemical analysis of a soil, though giving the 

 limits of the plant-food the soil can supply, often gives little 

 practical information as to its fertility or its need for manures. 

 It is necessary to know, as far as possible, the amounts of 

 the essential elements which are directly available for the use 

 of plants. One method, used to determine the amount of 



