FUNGOID PESTS OF TIMBER 143 



of stem and root, and is there used in the forma- 

 tion of new tissue, for a plant cannot grow with- 

 out food any more than an animal. The carbon 

 dioxide which a green plant takes up is derived 

 from the air, and it enters the plant through 

 minute pores on the leaves called stomata and, 

 to a lesser extent, through pores on the bark 

 called lenticels. On most leaves the stomata 

 are on the lower surface, a wise provision, for 

 in this position they are less likely to become 

 clogged by dirt. 



When a plant is attacked by a parasite one 

 or more of several things may happen. The 

 parasite may steal the food which rightly belongs 

 to and is necessary to the well-being of its host. 

 It may, if it be a root parasite, seriously inter- 

 fere with the absorption of water and dissolved 

 food material or, if it be a leaf parasite, it may 

 hinder the free passage of carbon dioxide. 

 Again, leaf parasites may partially or entirely 

 put a stop to the manufacture of sugar and 

 starch in the leaves. They may so injure the 

 tissues of the plant as to prevent the ascent of 

 raw food material or the descent of elaborated 

 food material. There are other and less obvious 

 results of fungoid attacks, but, in the main, 

 their injury is the result of impairing the func- 

 tions necessary to the growth of their host. 



Even the ecto-parasites, such as the mildews. 



