VEGETATIVE OBGAJfS Of PLANTS. 273 



having apparently, to a certain extent, a selective power. 

 See p. 401. 



3. The Root as a Magazine. In Fleshy Tap- 

 Roots, like those of the carrot, beet, and turnip, the 

 absorption of nutriment from the soil takes place princi- 

 pally, if not entirely, by means of the slender rootlets 

 which proceed abundantly from all their surface, and 

 especially from their lower extremities, while the older 

 fleshy part serves as a magazine in which large quantities 

 of carbhydrates, etc., are stored up during the first year's 

 growth of these biennial plants, to supply the wants of 

 the flowers and seed which are developed the second year. 

 When one of these roots, put into the ground for a sec- 

 ond year, has produced seed, it is found to be quite 

 exhausted of the nutritive matters which it previously 

 contained in so large quantity. 



Root Tubers, like those of the dahlia and sweet potato, 

 are fleshy enlargements of lateral or secondary roots 

 filled with reserve material, from which buds and new 

 stems may develop. Small tubers (Tubercles) are fre- 

 quently formed on the roots of the garden bean 

 (PJiaseolus). 



In cultivation, the farmer not only greatly increases 

 the size of these roots and the stores of organic nutritive 

 materials they contain, but, by removing them from the 

 ground in autumn, he employs to feed himself and his 

 cattle the substances that nature primarily designed to 

 nourish the growth of flowers and seeds during another 

 summer. 



Soil-Roots ; Water-Roots ; Air-Roots. We may 

 distinguish, according to the medium in which they are 

 formed and grow, three kinds of roots, viz.: soil-roots, 

 'water-roots, and air-roots. 



Most agricultural plants, and indeed by far the greater 

 number of all plants found in temperate climates, have 

 roots adapted especially to the soil, and which perish by 

 18 



