Workers in Plant Tissues. 



our knowledge has been considerably advanced, thanks 

 to the direct observations and experiments of modern 

 naturalists, there are many points which still remain 

 obscure. Up to two hundred years ago it was generally 

 believed that these galls were entirely vegetable pro- 

 ductions, and that the maggots found in them were 

 due to spontaneous generation : for it was an article 

 of universal belief throughout the Middle Ages that 

 all maggots in general arose from the various organic 

 substances in which they were found, by means of 

 that hypothetical process called spontaneous genera- 

 tion. The great anatomist Malpighi, however, aware 

 of the unsatisfactory nature of such a belief, began 

 closely to observe these galls, and his studies were 

 soon rewarded by indisputable evidence of their origin 

 being caused by the work of insects in fact, they arose 

 from punctures made in the tissues of the plant by the 

 Gall-fly in the process of depositing her eggs. But 

 we must not hastily jump to the conclusion that every 

 gall has been formed through the agency of a Gall-fly, 

 for there are a great variety of insects, as well as mites, 

 which form galls upon many plants, and such galls are 

 often utilized by certain species of Gall-flies, which, 

 cuckoo-like, deposit their eggs in nests provided by 

 other insects. Some of the Scale insects of Australia, 

 for instance, form extremely curious galls on the 

 eucalyptus trees, which are almost invariably inhabited 

 by the offspring of these parasitic Gall-flies. To find 

 out the exact manner in which many of these plant 

 galls originate, therefore, is by no means easy. Those 

 found on the oak, and the handsome mossy-looking 

 bedeguar galls on the wild and cultivated roses, have 



251 



