I50 



All Food Is Made by Green Plains unit hi 



Epidermal cell with beginning 

 of root hair 



Soil particles clinging 

 to root hairs 



regions would have exactly the same 

 composition of mineral substances. 



Ordinary soil contains much besides 

 minerals. It has varying amounts of dead 

 and partially decayed plants and animals. 

 The soil from the forest floor, called 

 humus, is particularly rich in this. And 

 all soil contains water; even soil that looks 

 and feels dry contains small amounts of 

 moisture. This soil water holds in solu- 

 tion whatever substances in the soil are 

 soluble. It is quite different from rain 

 water. Finally, soil contains varying 

 amounts of air. The nearer the surface 

 and the less closely packed, the more air 

 it contains. 



A close-up of roots. When you uproot 

 a plant it is impossible to see one very 

 important part of the root system, the 

 part that is most active. This part can 

 be seen only if the young roots grow in 

 water or on a moist surface under glass. 

 See Exercise 5. You will find that near 

 the end of each little root there is a deli- 

 cate fuzzy covering consisting of tiny 

 hairs, called root hairs. See Figure 173. 

 It is difficult to conceive of the large 



Root hair 



Fig. 174 (above) Three root hairs ifjagnified 

 Each hair is a single cell. 



Fig. 173 (left) Sprojiti/ig seeds. Each one has a 

 young root with root hairs. Where on each root 

 are the shortest root hairs? (blakiston) 



number of root hairs on one plant even 

 though only a part of each rootlet is 

 covered with them. On the rye plant re- 

 ferred to above there might be, accord- 

 ing to some estimates, 14 billion root 

 hairs with a total length of 6000 miles. 



Each root hair is an outgrowth from 

 and therefore a part of a single epidermal 

 cell. Like many other plant cells it has 

 only a thin layer of cytoplasm lining the 

 cell wall; the interior is completely oc- 

 cupied by a large vacuole. Figure 174 

 is a drawing of some of the epidermal 

 cells of a young root. 



Diffusion through root hair membranes. 

 In the last unit you read how the mole- 

 cules of all substances are in constant 

 motion, and how substances, especially 

 gases and liquids, intermingle or diffuse. 

 You learned that many substances diffuse 

 throu[rh a membrane which has no vis- 

 ible pores. You showed that water can 

 diffuse through the cell membrane of 

 plant cells, passing both in and out. 



Now let us consider what happens in 

 the root hair stretched out in the soil 

 and surrounded by soil water. Does the 



