CHAPTER XXXIII 



TRANSFER OF MATERIALS IN PLANTS 



203. Exchange of materials in living cells. One-celled plants 

 and animals that move about in the water constantly come into 

 a new environment, from which they get supplies of water, oxy- 

 gen, minerals, and food 

 (or food-making materi- 

 als). In larger animals 

 and plants the life of 

 the cells, or at least 

 of the innermost ones, 

 can be maintained only 

 by means of an internal 

 transportation system. 

 This brings to the cells 

 their supplies of food, 



FIG. 62. Vessels in a plant water, oxygen, etc., and 



carries from them, to be 

 removed to the exterior, 

 their waste products. 



204. The conducting 

 systems of a plant. In 



all those plants that have a body which is made up definitely 

 of root, stem, and leaf the diffusion of water and of dissolved 

 substances between parts of the body is supplemented by the 

 transportation of material in mass. We have seen that water 

 and salts absorbed by the root-hairs diffuse through the cells 

 of the root cortex and then move bodily through special 

 vessels (see p. 44). These sap-carrying vessels are of sev- 

 eral kinds some long, some short, some consisting of single 



174 



Section of squash stem, showing phloem vessels, 

 P/i, which conduct food down from the leaves, and 

 two kinds of wood vessels, W, which conduct water 

 and salts up from the roots. The two kinds of 

 wood vessels are those with spiral thickenings in 

 the walls and those with pitted walls 



