40 Introduction to Botany. 



soil to a depth of from ten to twenty feet, this habit mak- 

 ing it valuable for hay and pasturage in regions of scanty 

 rainfall. 



28. Path of Absorbed Substances. — The water and sub- 

 stances in solution pass from the root hairs toward the 

 center of the root, where they enter tubes (see Figs. lo and 

 42) which conduct them through the stem into the leaves. 

 The osmotic force in the root hairs is sufficient to lift the 

 water in the stem to a considerable height (compare Ex- 

 periment 51), but this force is not of itself sufficient to 

 carry the water up rapidly enough to supply the evapora- 

 tion from the leaves, nor high enough to reach the tops of 



tall trees. Figure 10 shows 

 the relation between the root 

 hairs and the water-conducting 

 tubes (called tracheal tubes be- 

 FiG. 14. cause of their resemblance in 



Lemna, a floating water plant. The appearance to the trachcE or 



slender roots, destitute of root hairs, windpipe), 

 grow down in the water. Slightly 



magnified. 29. Roots 01 Water Plants. — 



The roots of water plants are 

 much less extensive than those of land plants. In the 

 case of such plants as Lemna and Spiivdcla (see Fig. 14), 

 which float upon the water, the roots are few, short, and 

 unbranched, and destitute of hairs. Such plants do not 

 need an elaborate root system, since water and food sub- 

 stances dissolved in it are available without stint at all 

 times. 



30. Roots of Parasitic Plants. — Some plants have de- 

 veloped parasitic habits and attach themselves to other 

 plants by means of their roots, having no direct connection 

 with the soil, but depending upon their host plant for the 

 water and other necessary food materials. If the parasite 



