SECTION X. ROOT-TUBERCLES
Tubercle is a big word, but you ought to know how to pronounce it and what is meant by root-tubercles. We are going to tell you what a root-tubercle is and something about its importance to agriculture. When you have learned this, we are sure you will want to examine some plants for yourself in order that you may see just what tubercles look like on a real root.
Root-tubercles do not form on all kinds of plants that farmers grow. They are formed only on those kinds that botanists call legumes. The clovers, cowpeas, vetches, soy beans, and alfalfa are all legumes. The tubercles are little knotty, wart-like growths on the roots of the plants just named. These tubercles are caused by tiny forms of life called, as you perhaps already know, bacteria, or germs.
Fig. 23. Tubercles on Clover Roots
The specimen at the right was grown in soil inoculated with soil from an old clover field. The one at the left was grown in soil not inoculated]
Instead of living in nests in trees like birds or in the ground like moles and worms, these tiny germs, less than one twenty-five thousandth of an inch long, make their homes on the roots of legumes. Nestling snugly together, they live, grow, and multiply in their sunless homes. Through their activity the soil is enriched by the addition of much nitrogen from the air. They are the good fairies of the farmer, and no magician's wand ever blessed a land so much as these invisible folk bless the land that they live in.
Just as bees gather honey from the flowers and carry it to the hives, where they prepare it for their own future use and for the use of others, so do these root-tubercles gather nitrogen from the air and fix it in their root homes, where it can be used by other crops.
Fig. 24. Soy Beans and Cowpeas, Two Great Soil-Improvers
In the earlier pages of this book you were told something about the food of plants. One of the main elements of plant food, perhaps you remember, is nitrogen. Just as soon as the roots of the leguminous plants begin to push down into the soil, the bacteria, or germs that make the tubercles, begin to build their homes on the roots, and in so doing they add nitrogen to the soil. You now see the importance of growing such crops as peas and clover on your land, for by their tubercles you can constantly add plant food to the soil. Now this much-needed nitrogen is the most costly part of the fertilizers that farmers buy every year. If every farmer, then, would grow these tubercle-bearing crops, he would rapidly add to the richness of his land and at the same time escape the necessity of buying so much expensive fertilizer.
EXPERIMENT
Take a spade or shovel and dig carefully around the roots of a cowpea and a clover plant; loosen the earth thoroughly and then pull the plants up, being careful not to break off any of the roots. Now wash the roots, and after they become dry count the nodules, or tubercles, on them. Observe the difference in size. How are they arranged? Do all leguminous plants have equal numbers of nodules? How do these nodules help the farmer?