220 PASTORAL AND AGRICULTURAL BOTANY 



Nitrogen-Storing Plants. We have, therefore, two classes of agri- 

 cultural plants: nitrogen- storing plants and nitrogen- consuming plants. 

 The nitrogen-storing plants are those which can utilize the free atmospher- 

 ic nitrogen. These plants, if the soil is rich in nitrogen, or if the nodules 

 do not form on their roots, become as ordinary nitrogen-consuming plants, 

 i.e., they require nitrogen. When the seeds of clover, or some other legu- 

 minous species are planted, soon after the primary roots appear with their 

 root hairs, the nodule producing organisms (Bacillus (Pseudomonas} 

 radicicola) attracted perhaps chemotactically to the fine root hairs, pene- 

 trate the walls of these root hairs and enter through these cells into the mid- 

 dle cortex layers of the root. So many organisms enter, that they form a 

 long, slimy cord, almost hypha like. Here in the root cortex cells, the 

 microorganisms form nests, or pockets, that become filled with bacteria. 

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

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

 Here they remain, utilizing the free atmospheric nitrogen, when stimulated 

 by small amounts of carbohydrates, derived from the green host plant 

 until about the time of flowering of the host, when the bacteria begin 

 to undergo involution changes, enlarging considerably in size and assum- 

 ing S-shaped, or Y-shaped forms (bacteroids). After this, they are gradu- 

 ally absorbed by the green plants until the tubercle becomes empty a 

 mere shell. The nitrogenous material has been dissolved and utilized by 

 the leguminous plant in the formation of plant substance, or in the form of 

 reserve food within seeds and other parts of the green host. 



Types of Leguminous Nodules. Although the organism is the same in 

 all leguminous plants, viz. Bacillus radicicola, it exists in varietal forms, 

 which are peculiar to each of the important species of leguminosae being, 

 therefore, polymorphic, although occasional cross inoculations occur (Fig. 

 95). A recent study of a large number of genera of leguminous plants by 

 Spratt has shown that there are four general types of tubercles: I. The 

 Genisteae type in which the nodule is primarily spherical, with a spherical 

 meristern outside the bacteroidal tissue, which becomes localized at certain 

 parts, and thus the nodule acquires a very uneven surface and shape. The 

 vascular supply forms one broad zone across the base of the nodule, which 

 subsequently branches and produces a varying number of strands. The 

 bacteroidal tissue becomes separated into a number of. distinct areas 

 with a varying amount of sterile tissue between. Plants with this type 

 of nodules are woody. Many are shrubs, e.g., Genista, Ulex, Amorpha; 



