No. 7. DEPARTMENT OF AGRICULTURE. 217 



pendent of this supply of nitrogen, viz: the leguminosae, and can 

 utilize directly free atmospheric nitrogen. 



We have, therefore, two classes of agricultural plants: Nitrogen 

 Consuming Plants and Nitrogen Storing Plants. The nitrogen con- 

 suming plants can utilize the nitrogen only in one form and that 

 is as nitrates. There are four groups of these nitrogen consuming 

 plants, viz: root crops, v/hich include the potato, sweet potato, rad- 

 dish, turnip, beet, mangold wurzel, horse raddish and onion; leaf 

 crops, viz: kale, celery, lettuce, cabbage, salad plants in general; 

 fruit crops which comprise apples, pears, quinces, strawberries, cher- 

 ries, peaches, etc., and lastly the cereals, such as wheat, rye, barley, 

 oat, millet, maize and various other grasses grow^n for hay. All of 

 these enumerated crops need nitrogenous supplies for their best de* 

 velopment, because sugar, starch and other carbohydrates reserve 

 materials are only stored in the plant in considerable quantities when 

 nitrogen is present in efficient supplies. The proper storage of the 

 various carbohydrates can take place only when the storage cells 

 are supplied with the requisite amounts of nitrogen and potash. 

 If the plant is nitrogen hungry, such carbohydrate supplies are not 

 usually reserved. 



The second group of agricultural plants, the nitrogen storing, 

 comprise those plants that belong to the natural order leguminosae. 

 These plants, if the soil is rich in nitrogen, or if nodules do not form 

 on their roots, act as ordinary nitrogen consuming plants, i, e., they 

 require nitrogen, but in ordinary soils, the tubercle forming bacteria 

 are in sufficient numbers to cause the production of nodular swell- 

 ings on their roots. When the seeds of clover, or some other legu- 

 minous species are planted, soon after the primary roots appears 

 with its root hairs, the nodule producing organism Rhizobium legu- 

 minosarum, attracted perhaps chemotactically to the fine root hairs, 

 pentrates the wall of these root hairs by ferment action and enters 

 through these cells into the middle cortex layers of the root. So 

 many organisms enter at once, that they form a long slimy cord, 

 almost hyphae like. Here in the root cortex cells, the micro-organ- 

 isms form nests, or pockets, that become filled with bacteria. The 

 presence of these rod-shaped bacteria causes the formation of swell- 

 ings, tubercles or nodules on the roots of the leguminous plants. 

 Here they remain, utilizing the free atmospheric nitrogen until about 

 the time of flowering, when the germ cells begin to undergo involu- 

 tion changes, enlarging considerably in size and assuming S-shaped, 

 or Y-shaped forms. After this, they are gradually absorbed by the 

 green plants as protoplasmic and nitrogenous substance and this sub- 

 stance is conveyed to the actively growing points, or stored as nitro- 

 genous reserve supplies. The growth of these useful organisms in 

 the soil is stimulated by aeration, by a considerable amount of or- 

 ganic material, by proper soil drainage, so as to prevent the ac- 

 cumulation of water and liming, so as to overcome soil acidity. 



When the leguminous crop is mature, or before it is mature, it 

 may be plowed under as a green manure. Here by the process of 

 putrefaction, the organic nitrogen of the living plant is converted 

 into a form of nitrogen which through the agency of the nitrifying 

 organisms is again converted into a form available to the growing 

 green plant. Thus the nitrogen cycle is completed. Or, if the legu- 

 minous crop is consumed, it should be used on the farm and not 

 15 



