434 INSECTS AFFECTING VEGETATION 



our fungi are limited to a rather narrow range of host plants; thus 

 we may expect the potato Phytophthora to attack several plants of 

 the potato family (Solanaeeae) . The investigator proved this same 

 was true of the attacks of downy mildew (Plasmopara) upon a num- 

 ber of species belonging to the cucumber family (Curcurbitaceae) . 

 Since our cereal grains belong to the same great family as the grasses 

 (Gramineae), we expect, and find that there is a development of the 

 same diseases upon many of them and upon the grasses growing 

 near by. In this connection it must be remembered that clover and 

 alfalfa are not grasses, but legumes. 



The leaves of the host plant provided as they are with stomates 

 or breathing pores, minute openings through the epidermal covering 

 of the leaf, will be attacked through these openings. The spores of 

 parasitic fungi after germinating upon the leaf will likely gain en- 

 trance into the interior leaf tissues through these openings much 

 more readily than by actual boring through the leaf epidermis. 



These stomates are present in the leaf covering upon the outside 

 of all green leaves and in the epidermis of young growing shoots. In 

 addition to these stomates certain classes of plants such as the plants 

 of the mustard family (Cruciferae) , as cabbage, cauliflower, turnip, 

 also the grape, fuchsia, impatiens, etc., are provided with water pores 

 marginal openings through which the excess water of the plants is 

 excreted. These water solutions of various materials offer a means 

 of growth for organisms, especially of the minuter forms. From the 

 culture drops thus formed the parasite enters the leaf by the water 

 pores. One of the most destructive known diseases of plants is the 

 black-rot of cabbage, cauliflower, turnip, ruta-baga, etc. This is due 

 to a bacterium which gains entrance very largely through the water 

 pores just described. So we must bear in mind that the very avenues 

 of transpiration or excretion, so essential to plant growth, are made a 

 means of exposing the plant to the danger of parasitic invasion. 

 This is analogous to the exposure of human subject to diseases of the 

 respiratory organs. At every turn we find convincing evidences of 

 the mutual adaptation of parasitic fungi to their host plants, in 

 nothing more strongly marked than in the limitation of the species 

 of plants attacked by a given parasite as discussed in the beginning 

 of this paragraph. In view of the fact that so long as the leaves of a 

 plant continue to function as leaves, these natural openings will be 

 maintained, it will be seen that the risk of exterior infection from 

 parasitic fungi is continuous for any given plant ; it lasts for its whole 

 growing period. 



THE PLANT'S PROTECTION AGAINST PARASITES. 



In the case of woody growths we have the development of corky 

 epidermis or bark which seems primarily designed to protect the in- 

 terior, living layer from invasions of this sort. In a similar manner 

 the external layer or bark of all growing plants, including herbs, is 

 provided with a protective covering or epidermis. The skin of the 

 apple or of the grape and the covering of the potato stem are all fa- 

 miliar and serve this function of protection to the inner tissues. In 

 young plants there is retained the power of protective growth in re- 



