Nutrition of Multicellular Plants - 245 



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Fig. 13-9. Roots of a white elm exposed by the en- 

 croachment of a stream. (Courtesy of the Missouri 

 Botanical Garden.) 



effective in the absorption of water and salts. 

 In fact absorption is confined largely to the 

 root hair zone, which lies above the growing 

 point in each of the many young branches or 

 rootlets (Fig. 13-10). 



The root hair zone displays a highly or- 

 ganized structure, as may be seen in cross 

 section (Fig. 13-11). At the center lies the 

 vascular cylinder, sheathed by a thick cortex. 

 The vascular cylinder is composed chiefly of 

 ducts and sieve tubes, surrounded by a single 

 layer of growth tissue, the pericycle. The 

 thickness of the cortex is due to the several 

 layers of colorless parenchyma, which is cov- 

 ered externally by a thin epidermis from 

 which the root hairs are derived. 



Absorption by the root is determined 

 partly by osmosis, although active transport 

 mechanisms (p. 121) serve to maintain a high 

 concentration of certain ions in the tissues of 

 the root. Water from the soil enters the root 

 hairs and then passes across the cortex tissues, 

 entering the ducts of the vascular cylinder 

 (Fig. 13-11). The root hairs extend out from 

 the epidermis and make contact with the film 

 of water that wets the particles of moist soil 



(Fig. 13-12). Although only 1 to 2 centimeters 

 in length, each root hair zone displays a 

 prodigious number of these tiny structures. 

 The number of root hairs in a single rye 

 plant, for example, has been carefully esti- 

 mated at more than 14 million, adding some 

 4 thousand square feet of extra surface to 

 the absorbing area of the root system. The root 

 hairs, being very delicate, dry out and die 

 within a few minutes if they are exposed to 

 air. Consequently, if a plant is to be trans- 

 planted during the growing season, the soil 

 around the roots must be disturbed as little 

 as possible. This care avoids tearing the root 

 hairs — which may be cemented to the soil 

 particles — and protects them from being 

 killed by drying. Without root hairs a plant 

 usually wilts and dies before new root hairs 

 can be regenerated, because the root hairs 

 account for a high percentage of the absorb- 

 ing surface of the root. 



Soil water is a hypotonic solution of inor- 

 ganic salts, which yields water to the root 

 hair cells; and as these epidermal cells take 

 in water, they become hypotonic to the cells 



Fig. 13-10. Root hairs as seen on a seedling of the 

 garden cress grown in moist air (X 4). 



