286 The Phenomena of Morphogenesis 



K. M. Smith (1920) had shown that the damage to leaves of apple by 

 capsid bugs was caused by secretions of the salivary glands, and this sug- 

 gested the possibility that the glands of the gall-producing larva secrete 

 an enzyme which calls forth a specific growth reaction in the host tissue. 

 Parr demonstrated the presence of enzymes in the gland extract but found 

 that these did not stimulate gall formation when sterilized. Martin 



T 



Fig. 11-8. Section through portion of an insect gall showing modification of normal 

 leaf structure (left). The larval chamber is surrounded by mechanical tissue. (From 

 Kiister. ) 



(1942), however, produced abnormal growth in sugar cane with steri- 

 lized extracts. 



Substances produced by the growing larva rather than those injected 

 with the egg are probably most important in gall formation. Little is 

 known about these substances, however, or the place and manner in which 

 they are introduced into the tissues of the plant (Kostoff and Kendall, 

 1929). 



A morphological problem of some interest is whether these galls and 



