AIDED BY OTHER ORGANISMS 



27 



unsuited for certain plants and also how the farmer may injure 

 his crops by using too much of certain fertilizers or by the im- 

 proper mixtures of the fertilizers with the soil. The root-hairs 

 persist for a very short time and then perish, but new ones are 

 produced near the growing tips. The number of root-hairs 

 varies with the amount of water in the soil. They are more 

 numerous in dry than in wet soils. (Figs. 17, 18 and 19.) 



FIG. 18. Showing attachment of root- 

 hair to epidermis. 



FIG. 19. Showing relation of root-hair to 

 soil particle. 



Aided by Other Organisms. Plants are frequently facil- 

 itated in securing their food by other organisms. Dead plants, 

 dead animals and manures must undergo a decay before they are 

 sources of available food for most plants. This decay is caused 

 by bacteria and fungi. (Page 172.) Certain bacteria also aid 

 the plant by taking the nitrogen from the air and fixing it 

 in such' a manner as to make it available for plant food. 

 (Page 117.) 



