STUDIES OF SEED-PLANTS 161 



the soil ; but the mangrove trees mentioned above often grow 

 in mud covered with water, which interferes with aeration. 

 Certain species of mangroves overcome this difficulty by 

 the development of branch roots which grow up into the 

 air out of the mud and water. These roots have special 

 air-passages for conducting air down into the roots, and 

 they take up oxygen and discharge carbon dioxide as do 

 other plant tissues exposed to the air; that is, they breathe. 



(5) Parasitic Roots. Many species of plants have 

 peculiar roots adapted for penetrating the epidermis of other 

 plants and thus forming a close attachment for absorbing 

 the necessary foods. Examples are the mistletoe, which 

 grows on the bark of oaks, apple, and other trees ; and the 

 common dodder, which has root-like branches penetrating 

 other plants from which it can absorb suitable foods. These 

 are cases of plant parasites; and this condition in which 

 one plant (the parasite) absorbs its food from another plant 

 (the host) is parasitism. The mistletoe has pale green leaves 

 and can make some starch, so it is only in part a parasite. 

 The dodder has no roots in the soil, only some very small 

 scale-like rudiments of leaves without chlorophyll, and a yel- 

 low, string-like stem with very little chlorophyll. Evidently 

 it must depend upon other plants for the foods which ordinary 

 plants get from the soil and make in their green leaves. In 

 recent years the dodder has been accidentally planted in 

 many fields by seed mixed with those of clover and alfalfa, 

 and has caused great damage by absorbing food from the 

 plants with which it makes a root-like connection. For 

 more facts concerning the relation of this weed to agriculture, 

 read Farmers' Bulletin 306, " Dodder and its Relation to 

 Farm Seeds." 



154. Binding Action of Roots. After any heavy rain- 

 storm one can see that bare soil, such as on roads and culti- 

 vated fields, has been washed away where water has flowed 

 rapidly, while grassy sod has not been so affected. This 



