poor in nitrogen, to obtain nitrogen from some source other than the soil in which they grow.  A pea plant in soil that contains no nitrogen products and watered with water that contains no nitrogen, will, after sprouting and growing for a length of time, be found to have accumulated a considerable quantity of fixed nitrogen in its tissues The only source of this nitrogen has been evidently from the air which bathes the leaves of the plant or permeates the soil and bathes its roots This fact was at first disputed, but subsequently demonstrated to be true, and was found later to be associated with the combined action of these legumes and certain soil bacteria.  When a legume thus gains nitrogen from the air, it develops upon its roots little bunches known as root nodules or root tubercles.  The nodules are sometimes the size of the head of a pm, and sometimes much larger than this, occasionally reaching the size of a large pea, or even larger.  Upon microscopic examination they are found to be little nests of bacteria In some way the soil organisms (Fig 27) make their way into the roots of the sprouting plant, and finding there congenial environment, develop in considerable quantities and produce root tubercles in the root.  Now, by some entirely unknown process, the legume and the bacteria growing together succeed in extracting the nitrogen from the atmosphere which permeates the soil, and fixing this nitrogen in the tubercles and the roots in the form of nitrogen compounds.  The result is that, after a proper period of growth, the amount of fixed nitrogen in the plant is found to have very decidedly increased (Fig 25 E).

This, of course, furnishes a starting point for the reclaiming of the lost atmospheric nitrogen.  The legume continues to live its usual life, perhaps increasing the store of nitrogen in its roots and stems and leaves during the whole of its normal growth.  Subsequently, after having finished its ordinary life, the plant will die, and then the roots and stems and leaves, falling upon the ground and becoming buried, will be seized upon by the decomposition bacteria already mentioned.  The nitrogen which has thus become fixed in their tissues will undergo the destructive changes already described.  This will result eventually in the production of nitrates.  Thus some of the lost nitrogen is restored again to the soil in the form of nitrates, and may now start on its route once more around the cycle of food.

It will be seen, then, that the food cycle is a complete one.  Beginning with the mineral ingredients in the soil, the food matter may start on its circulation from the soil to the plant, from the plant to the animal, from the animal to the bacterium and from the bacterium through a series of other bacteria back again to the soil in the condition in which it started.  If, perchance, in this progress around the circle some of the nitrogen is thrown off at a tangent, this, too, is brought back again to the circle through the