OTHER WAYS OF PROCURING FOOD 69 



But by far the most troublesome plant diseases are produced by 

 parasites which attack economic plants and thus cause millions 

 of dollars of damage annually. There are four distinct kinds of 

 injury produced by parasites: 



1. While not causing any especial trouble of a localized nature, 

 they may weaken the host to such an extent by reducing its food 

 supply, that it cannot resist unfavorable conditions such as drouth, 

 cool weather, etc. The food supply is so nearly exhausted and so 

 small a surplus can be maintained that the first onset of unfavor- 

 able conditions results in the death of the host. 



2. Some parasites stimulate local growth so that large abnormal 

 outgrowths or excrescences develop. The " witches brooms" so 

 common on hackberry (Celtis) trees, peach curl, and black knot 

 of plum are examples of such growths produced as the result of 

 attacks by parasites. 



3. Some parasites, such as the one causing fire blight of pear, 

 produce toxic substances which, when present in the tissues, cause 

 the death of the parts affected. Wherever this disease has gained 

 a foothold, the leaf dies and ultimately the tree. 



4. Other parasites bring about the death of the host by inter- 

 fering with some general and necessary function such as the water 

 conduction. By plugging up the vessels of the wood which carry 

 the water from the roots to the leaves, the general water supply 

 from the soil is shut off and the plant dies from lack of water. 

 Although in the past this has been supposed to be the action of 

 the so-called "wilt" diseases, it is now known that the majority of 

 these diseases produce the death of the plant through the pro- 

 duction of toxins. They thus belong in group (3); but this does 

 not preclude the possibility that a part of the damage done in 

 some cases may be connected with the water supply. 



Mixophytes. — In a previous section mention has been made 

 of the possibility of manufacturing food and at the same time 

 using food already elaborated. There is no good reason why any 

 green plant should not be able to use organic food if supplied to 

 it in solution or if it possesses enzymes capable of making the 

 food available; and some plants have special structures to enable 

 them to supplement their normal supply of organic food, espe- 

 cially nitrogenous compounds. Such are the insectivorous plants, 

 which include the pitcher plants (Sarracenia and Nepenthes), 

 the sundew (Drosera), Venus's flytrap (Dionsea), and the bladder- 



