ROOTS AND THEIR WORK 97 



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

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

 or in other jikiees where they ean Ret ii fa\<nlable foothold. Sueh plants 

 are called epiyi/fi// is, or air plants. The tropieal orchid seen in our green- 

 houses is an I'xample. K.xamine the roots of sueh a plant. Notice how 

 thick thej' are. They are usually provided with a spongy (issue around 

 the outside which has the function of absorbing water. 



Parasitic Roots. — .1 few plaids lire on other living planlx, and develop 

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

 parasite. The plant or animal on wliich 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 



ELESIENTARY 



Sharpe, A Laboratory Manual for the Snlulion of Problems in Biology. American 



Book Company. 

 Andrews, Botany aU the Year Round, ChapivII. American Book Company. 

 Atkinson, First Studies of Plant Life, Chipa.^IX, XI, XII. Ginn and Company. 

 ■ Coulter. Plant Studies, Chap. V. D. ApjJeton and Company. 

 Goff and Mayne, First Prinql^les of Arr'uiuUure. American Book Company. 

 Moore, The Physiology of Man and Olhir Animals, Henrj' Holt and Company. 

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



f 



ADTANCED 



Coulter, Barnes, and Cowles, A Tetlbc^k 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 27r')0, 56-04. American Book Company. 

 Green, Vegetable P/i!/sioZWrChaps. V, VI. J. and A. Churchill. 

 Farmers' Bulletin SO, U.oT Department of Agriculture. 

 Kerner-Oliver, Xatural History of Plants. Henry Holt and Companj-. 

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

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



