HOW DISEASES ARE CAUSED 



301 



There are many kinds and degrees of parasitism. The dodder, 

 a plant of the morning-glory family, often grows as a climbing 

 vine on various common shrubs. It digs its roots into the stem 

 of the host and absorbs from it organic food as well as water 

 and salts. When the 

 dodder is deprived 

 of food, however, it 

 is capable of form- 

 ing chlorophyl and 

 leading an inde- 

 pendent life. In 

 other words, some 

 parasites are such 

 whenever they have 

 a chance to be and 

 are independent or- 

 ganisms when they 

 must get food more 

 actively. The mis- 

 tletoe, another par- 

 asite that lives on 

 branches of trees, 

 produces chloro- 

 phyl and depends 

 upon the host only 

 for water and salts. 

 The common mos- 

 quito, of which there 



Fig. 144. Tubercles on the roots of red clover 



The swellings are inhabited by a vast number of tiny 

 one-celled organisms that feed upon carbohydrates pro- 

 duced by the clover plant. These guests absorb nitro- 

 gen from the air and combine it with material taken 

 from the host, producing proteins. The clover plant 

 makes use of the excess of protein. There is thus a 

 partnership in which both members benefit 



are several species 



living in our country, catches tiny water plants and animals dur- 

 ing its early stages ; but after reaching the adult stage the fe- 

 male mosquito is a parasite in that she draws upon the juices of 

 larger organisms for food. Thus we see different degrees of food 

 parasitism: there are what we may call (i) compulsory as 

 against optional parasitism; (2) complete as against partial 

 parasitism; and (3) permanent as against ^gw/>orarv parasitism. 



