36 



DICOTYLEDONS 



on the tiny root-ends of the plants of this order and grow as 

 parasites on them, being nourished by their juices. They cause 

 those parts of the root on which they settle to grow exuberantly, 

 thus forming little nodules or tubercles (fig. 32, C and ^) on the 

 roots, isow, these bacteria, which are themselves tiny plants, 

 have the power, unlike other plants, of taking in, from the air, 

 nitrogen which is an essential ingredient of the living parts of 

 plants and without which plants cannot thrive. As other plants 

 cannot take nitrogen from the air which, indeed, always contains 

 plenty of it, they must take it from the soil through their roots. 

 These bacteria, then, take their supply of nitrogen from the 

 air. When, eventually, they die, their remains serve the Gram 

 plant, or any other plants that may be grown in that soil, as 

 very good manure containing, as they do, plenty of nitrogen. 

 We see here, then, a beautiful reciprocity. At lirst, the Gram 

 plants allow the bacteria to settle on them and to participate 

 in the food they draw from the soil and air for themselves. The 

 guest in return gives nitrogen to the host. 



This fact is of the greatest importance to agriculture. With 

 each crop the farmer takes away from the 

 field a great quantity of nitrogen deposited 

 in the seeds and other parts of the crop, 

 and this nitrogen had all been gained from 

 the soil. If he wants another good crop 

 next year, he must needs replace what he 

 has taken away, and this he does in the 

 shape of manure. If he also grows such 

 plants as Gram, Lentils, Peas, etc., in the 

 cropped fields, these themselves will help 

 to manure the soil hy acting as hosts to 

 the bacteria which absorl) nitrogen from 

 the air. 



4. The Flower has some resemblance 

 to a butterfly (fig. 34). It is irregular, but a vertical section 

 divides it into symmetrical halves. We call such flowers bilateral- 

 ly symmetrical or zygomorphic (from (ireek zygon, yoke, and 

 morplie, shape). 



Fig. :J4 — Buttertty-flower 



of the Pea. 



a. Calyx, b. Standard. 



c. Winjfs. d. Keel. 



