It is not enough for water, with mineral substances in solution, merely to enter the root-hair. As far as the plant is concerned its journey is only now begun. A chain of progress is started, exactly as in the potato that holds a solution of sugar. The cell-sap in the root-hairs becomes weaker as a result of this absorption of weak solutions from the soil. The sap of a neighbouring cell, towards the interior of the root, is therefore stronger than this. Thus solutions originally absorbed by the root-hair travel, as a result of the osmotic force thus set up, to the inner region of the root. Here they enter the vessels of the wood, but not as a result of osmosis because the vessels are dead elements. The solutions are in some way forced into their cavities as a result of pressure. Having entered one vessel, the absorbed solutions can pass to others through thin places in the vessel walls. In these vessels of the wood the sap then rises. Exactly how it does so is one of the many unanswered problems of science.
The Selective Power of Protoplasm
Protoplasm in some way exercises a selective power of absorption and allows certain substances to enter the root- hairs more freely than others. Those that the plant makes use of are quickly removed from the root-hairs, which then absorb still more of these particular substances.
It is on this fact that the rotation of crops depends. Hundreds of years before there was a Science of Botany, agriculturalists refrained from growing the same crop on the same land two years in succession. By varying the crops better results were gained. The scientific reason for this is that one crop, X, let us say, absorbs certain substances, ABC, in excess and so impoverishes the ground as far as these particular substances are concerned. Another crop, Y, requires but little of ABC but absorbs other substances much more freely. During Y’s tenure of the soil, therefore, the substances ABC are gradually re-accumulating, with the result that, after a certain interval of time, if the crop X be grown once more on this plot of land, ABC are again present in sufficient proportion to supply its need.
Rotation of Crops
In different parts of the country slightly different rotations are in practice. A four-year cycle is common in the Eastern Counties. Clover is followed by Wheat ; Turnips or some other root crop follow ; and in the fourth year Oats or Barley are harvested.
This is the Norfolk Rotation. If every fourth year is not assigned to Clover, some other Leguminous crop takes its place. The Clover is harvested for fodder, but the roots are ploughed into the soil, thereby greatly increasing its nitrate content.
This increase depends upon the association of the roots with bacteria ; a partnership somewhat analogous to that of Beech and Heather with the mycorrhiza-forming fungus.
The bacteria enter Leguminous roots from the soil by way of root-hairs. In the cortex of the root they multiply rapidly. To accommodate the colonies of bacteria, the tissues of the roots grow in such a way that nodules, or tubercles, are formed in numbers along the main root and its branches.
These nodules occur on the roots of very many Leguminous plants that grow in the earth. They are not present on the Broad Bean roots in the experimental gas-jars, because the invading bacteria are absent from the water.
The enriching of the land depends upon the power of these particular bacteria to make the free nitrogen of the air in the soil combine with other elements, so that ultimately nitrates are abundantly formed. Their mode of action is by no means understood, but the result of their activity has been demonstrated repeatedly. Soil in which Lupin or Clover has grown has been found, upon analysis, to be much richer in nitrates at the end of the growth period than it was at the beginning, in spite of the fact that the roots have been absorbing nitrates during the whole period.