MANUAL OF GENERAL AGEICULTUEE. 39 



large enough for the roots of the previously germinated 

 Canada field peas to go through. Select 100 of the most 

 vigorous plants and arrange ten in each tumbler so that 

 the roots are below and tops above the covers. In about 

 two weeks it will be necessary to make a wire frame to 

 support the plants. Place the tumblers in a sunny place 

 and allow the culture to grow four weeks. During this 

 period every few days add distilled water so that the 

 quantity of the solution remains about the same. 



Make notes weekly concerning the general vigor of 

 the cultures. Final notes should include (1) Average 

 length of tops and of roots of each culture, (2) Green 

 weignts of tops and of roots. 



36. NITROGEN NODULES. 



Go out and dig up (without injury to the roots) a 

 specimen of as many of the following as are conveniently 

 near: a bean, a pea, a clover, an ailaila, a lupine. These 

 plants all belong to the same family, "leguinmosge," and 

 are commonly called "legumes." Examine the roots for 

 very small knots called "nodules." Also dig up the 

 roots of some cereals and examine them for nodules. Do 

 you find any? Draw one root system showing nodules. 



The nodules are the home of bacteria. (See Exercise 

 65, first paragraph.) These minute organisms are able 

 to use the free nitrogen of the soil air and combine it 

 with the mineral matter of the soil to form nitrates. The 

 plant cannot use free nitrogen but flourishes on the ni- 

 trates which the bacteria offer in return for the home pro- 

 vided by the legume. The plant satisfies its needs from 

 the nitrates thus produced and any excess remains in the 

 soil for future crops. This suggests a reason for crop 

 rotation practice of following a legume crop (a nitrogen 

 food producer) by a cereal crop (a nitrogen food con- 

 sumer.) 



