PLANT-EATING INSECTS 201 



ovipositors, or as a varnish to seal up the injury — 

 certainly not to poison the tissue. Various careful 

 observations have proved (in the case of the true gall- 

 wasps of the family Cynipidce) that the growth of the gall 

 structure does not commence until after the advent 

 of the larva, even though hatching may be postponed for 

 a considerable period ; and that the stimulus to unhealthy 

 growth on the part of the plant is furnished by the 

 gnawings, possibly also by the secretions, of the tiny grubs. 

 With the saw-flies, as we have seen, the growth of the gall 

 is complete, or far advanced, before the egg hatches; but 

 in these cases it is known that the egg itself undergoes 

 changes, and increases in size, and thus probably supplies 

 the irritation which determines the production of a gall. 



At first thought it may seem incredible that the mere 

 swelling of the egg, or the gnawing of a microscopic grub, 

 should supply the impetus to abnormal growth. But the 

 egg is invariably laid in contact with what botanists term 

 cambium or meristem tissue — that is to say, the particular 

 layers of vigorous cells whose active multiplication by 

 fission brings about the phenomenon which we call " the 

 growth of the plant." These cells are extraordinarily 

 sensitive, and the presence of foreign bodies — i.e. the 

 eggs or grubs — excites them to irregular and excessive 

 growths. This much we know. But why galls invariably 

 " come true " remains a mystery. For each of the many 

 kinds of gall insects known to science is bred from a 

 perfectly distinct and characteristic gall. Take for 

 example the three familiar kinds formed on the leaves of 

 the wild rose by members of the genus R/wdites. The 

 three gall-wasps concerned are closely related. Yet the 

 galls from which they come are widely different. First 

 there is the familiar " bedeguar," or " robin's pin-cushion," 

 the work of the little gall-wasp Rhodites rosce. It is 



