ROOTS AND THEIR WORK 97 



Air Roots. In tropical forests, where the air is always warm ami 

 moist, some plants have come to live above the soil on the trunks of trees, 

 or in other places where they can get a favorable foothold. Such plants 

 are called epiphytes, or air plants. The tropical orchid seen in our green- 

 houses is an example. Examine the roots of such a plant. Notice how 

 thick they are. They are usually provided with a spongy tissue around 

 the outside which has the function of absorbing water. 



Parasitic Roots. A few plants live on other living plants, and develop 

 by the aid of nourishment taken at their expense. Such a plant is called a 

 parasite. The plant or animal on which the parasite lives is called the host. 

 The mistletoe is an example of a parasitic plant. An examination of its 

 roots shows that they have bored their way into the stem of the host. 

 These roots not only penetrate the bark, but push toward the center of the 

 tree, taking nourishment from the cells there. The dodder is another 

 seed-bearing plant which has this habit. Dodder produces from seed, but 

 is unable to live alone after it has passed the seedling stage, and will die if 

 it cannot find a suitable host. It is found on many common weeds, as 

 jewelweed and goldenrod. Many of the lower plants live as parasites, 

 among them being mildew, rusts, and smuts found on roses, grain, and 

 corn. 



REFERENCE BOOKS 



ELEMENTARY 



Sharpe, A Laboratory Manual for the Solution of Problems in Biology. Ameri :u: 



Book Company. 



Andrews, Botany aU the Year Round, Chap. II. American Book Company. 

 Atkinson, Firxl >'/*////, of Plant Life, Chaps. IX, XI, XII. Ginn and Company. 

 Coulter, Plant Studies, Chap. V. D. Appleton and Company. 

 Goff and Mayne, First Principles of Agriculture. American Book Company. 

 Moore, The Physiology of Man a \ nimals, Henry ITolt and Company. 



Stevens, Introduction to Botany, pages 31-44. D. C. Heath and Company. 



ADVANCED 



Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Com- 

 pany. 



Detmer-Moor, Practical Plant Physiology. The Macmillan Company. 

 Goodale, Physiological Botany. American Book Company. 

 Gray, Structural Botany, pages 27-39, 56-64. American Book Company. 

 Green, Vegetable Physiology, Chaps. V, VI. J. and A. Churchill. 

 Farmers' Bulletin 86, U.S. Department of Agriculture. 

 Kerner-Oliver, Natural History of Plants. Henry Holt and Company. 

 MacDougal, Plant Physiology. Longmans, Green, and Company. 

 Pfeffer, W., The Physiology of Plants. Clarendon Press. 



HUNT. ES. BIO. 7 



