INSECTS AND DISEASES 



99 



grow by sending out germ tubes. These tubes enter the leaf 

 — some through open stomata only, others by penetrating 

 the epidermis. If the leaves are covered with a spray con- 

 taining a compound poisonous to the germinating spore, It 

 will be killed before entering the leaf. The spores germi- 

 nate when the leaf has a moist surface from which the poison 

 can enter the thin-walled tube of the spore. The importance 

 of complete coverage of the leaf is evident when the stomatal 



Fig. 16. Stomatal penetration of a sugar-beet leaf by the hyphae of Cercospora 

 beticola Sacc. (From Melhus and Kent's Elements of Plant Pathology. By permission 

 of The Macmillan Co., publishers, 1939.) 



distribution Is considered. Reference to the table on page 

 seventy-five shows that because of the stomatal distribution 

 of thousands per square inch and, for most plants In the 

 garden, on both surfaces, it Is very Important to cover all the 

 leaves completely on both surfaces with a material that will 

 spread over every cell. It is necessary to spray frequently as 

 long as new leaves continue to develop, In order to protect 

 the whole plant from the entrance of diseases (Fig. 16). 



The damage caused by animal parasites that chew and 

 eat the leaves Is more obvious than that caused by such 



