440 INSECTS AFFECTING VEGETATION 



show other instances of survival under many various and instructive 

 conditions. 



HOW THESE PARASITES ROB THE HOSTS. 



There is an old saying about the stable door and the stolen 

 horse ; similar application may be made for plants and parasitic fungi 

 in a manner which we shall presently perceive. To obtain food we 

 must reach the source of supply; the manner of reaching it is less 

 important than the result. Now it occurs that cultivated and wild 

 plants of the higher classes are wrapped about by a covering of skin 

 or bark, and the food-filled juices are within ; to feed upon any living 

 host the parasite must gain access to the internal tissues of that 

 host. It so happens that there are minute openings or stomates 

 (breathing pores) through the skin of leaves and of young green 

 stems ; these openings are as necessary as the stable door, and through 

 them the thief may enter. Were these openings to become entirely 

 closed the plant would languish, and remaining open, they constantly 

 offer a way for the tender tip of the growing germ thread of a fun- 

 gus to push its way through the plant covering and to luxuriate 

 within the host upon the substance of the plant. Once within, the 

 fungus thrives, rapidly multiplies its branches, and if in summer, 

 commonly thrusts its fertile threads through some of these breathing 

 pores to bear its spores outside where they may become more widely 

 distributed than if remaining within the tissues of the host plant. 

 Should, however, the winter season be near, resting spores may be 

 formed, or their formation be provided for within the leaves, or 

 diseased parts, as in grape downy mildew, elm-leaf disease and in 

 black-knot of plum and cherry. Thus the cycle of development 

 continues indefinitely unless some agency intervene to destroy the 

 spores, to prevent their germination, or the parasite itself so exhaust 

 the host plant as to destroy it entirely and the fungus perish for lack 

 of suitable nidus. However, this rarely occurs, not perhaps, so often 

 as men are guilty of killing the goose which lays the golden egg. 

 Herein, we meet another fact, namely, that parasitic fungi of a 

 given kind are limited to a particular host plant of a certain species, 

 or to a small number of related plants, so that if a congenial host is 

 lacking the fungus will not thrive. 



The fungus thread growing within any plant will not flourish if 

 simply passing between the cells of the host; penetrating organs 

 pierce the cell walls and are able to absorb nutriment from the cell 

 interior. The diverse forms of sucking organs, and the peculiar 

 structures of fungus threads in these situations would in them- 

 selves require much study and investigation to present them properly. 

 We must further conceive that a fungus may often penetrate the 

 bark of a tree for example if aided by rifts caused by freezing or 

 similar disturbances, to say nothing of the openings offered by 

 wounds, the breaking of branches, etc. A recent illustration of the 

 danger of rifts in the bark of trees is offered by the chestnut disease 

 which is proving so destructive near New York City. Few parasitic 

 fungi have that penetrating power of thrusting the haustoria through 

 the plant covering such as we find in the case of the dodder that 



