“It listens like it, as the Swedes say,” said Percy, “but the advertisements of these germ cultures put out by commercial interests are usually very misleading.  The safest and best and least expensive method of inoculating a field for alfalfa is to use infested soil taken from some old alfalfa field or from a patch of ground where the common sweet clover, or mellilotus, has been growing for several years.  I saw the sweet clover growing along the railroad near Montplain, and there is one patch on the roadside right where—­when you enter the valley on the way to the station.”

“Right where Adelaide smashed that nigger’s eye with her heel and helped Mr. Johnston capture them both,” broke in the grandmother.  “That’s the only good thing I can say for her peg heeled shoes.”

Adelaide colored and Percy now understood what had been a puzzle to him.

“The same bacteria,” he went on quickly, “live upon both the sweet clover and the alfalfa, or at least they are interchangeable.  These bacteria are not a fertilizer in any ordinary sense, but they are more in the nature of a disease, a kind of tuberculosis, as it were; except that they do much more good than harm.  They attack the very tender young roots of the alfalfa and feed upon the nutritious sap, taking from it the phosphorus and other minerals and also the sugar or other carbohydrates needed for their own nourishment, since they have no power to secure carbon and oxygen from the air, as is done by all plants with green leaves.  On the other hand, these bacteria have power to take the free nitrogen of the air, which enters the pores of the soil to some extent, and cause it to combine with food materials which are secured from the alfalfa sap, and thus the bacteria secure for themselves both nitrogen and the other essential plant foods.  The alfalfa root or rootlet becomes enlarged at the point attacked by the bacteria, and a sort of wart or tubercle is formed which resembles a tiny potato, as large as clover seed on clover or alfalfa, and, singularly, about as large as peas on cowpeas or soy beans.  On plants that are sparsely infected, these tubercles develop to a large size and often in clusters.  While the bacteria themselves are extremely small and can be seen only by the aid of a powerful microscope, the tubercles in which they live are easily seen, and they are sufficient to enable us to know whether the plants are infected.”

“I wish you would tell me the difference between the words inoculated and infected,” said Adelaide.

“Inoculated is used in the active sense and infected in the passive,” said Percy.  “Thus the red clover growing in the field is infected if there are tubercles on its roots, although it may never have been inoculated; and we inoculate alfalfa because it would not be likely to become infected without direct inoculation.”

“Under favorable conditions,” continued Percy, “these bacteria multiply with tremendous rapidity, somewhat as the germs of small pox or yellow fever multiply if allowed to do so.  A single tubercle may contain a million germs which if distributed uniformly over an acre would furnish more than twenty bacteria for every square foot.”