ROOTS AND THEIR WOUK 97 



Some plants, as strawberry, couch grass, and many others, develop new 

 plants by striking root at any point on the reclining stem where it touches 

 the ground. This fact is made of use by practical gardeners in the layering 

 of plants. 



Examine the Indian corn for another kind of adventitious root. Here 

 they serve as props for the tall stem. In the young seedlings of corn, notice 

 how early these roots develop. Also notice the manner in which they arise 

 on the stem. 



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 places where they can get a favorable foothold. Such plants are called 

 epiphytes or air plants. The tropical orchid seen in our greenhouses 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 or animal 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 jewel 

 weed and golden-rod. Many of the lower plants live as parasites, among 

 them being mildew, rusts, and smuts found on roses, grain, and corn. 



Reference Books 

 for the pupil 



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

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

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

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

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



FOR THE TEACHER 



Goodale. Physiological Botany. American Book Company. 



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



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



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



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



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



hunter's BIOL. — 7 



