396 GENERAL PLANT PATHOLOGY 



are found, but in which new kinds of tissues are formed differing from 

 the normal and in which definite proportions of form and size normal for 

 the species are repeated in them. Therefore, prosoplasms display in 

 their external form, something independent and well defined from the 

 organs of the normal plant both internally and externally. Hyper- 

 plastic tissues of this sort have been found until now only in the excres- 

 cences caused by parasites and almost entirely those of the animal 

 world, which produce zoocecidia. Six different orders of insects are the 

 principal producers of galls and various fungi. They are as follows: 

 The Acarina, or Mites of diminutive size, produce galls of simple form 

 and structure. 



The Diptera, or Flies, cause many prosoplasms. The galls produced 

 by the gall gnats, or gall midges, are very different in character and often 

 very complicated. 



The Hemiptera, which include the aphides commonly known as 

 green fly and plant lice, also produce numerous usually simple proso- 

 plasms. 



The Hymenoptera, or gall-wasps, produce striking galls on account 

 of their size, diversity and complexity of form external and internal. 



The Coleoptera and Lepidoptera (Heterocera) are responsible for 

 relatively few galls, and if formed their structure is relatively simple. 



There are several plant-produced galls, or mycocecidia, in which 

 there is a regular arrangement of certain elements such as the cells in 

 which anthocyanin is formed. Ustilago Treuhii causes the production 

 of canker-like excrescences on the stems of Polygonum chinense, which 

 consist of spongy, parenchymatous, wood tissue. The excrescences, 

 which develop from the canker swelling, are fleshy, succulent, easily 

 breakable, irregularly bent, cyhndric and often longitudinally furrowed 

 broadened at the top Hke the head of a snail. The fruit galls, which 

 represent the part which produces the spores of the fungus, are repre- 

 sented by this part of the gall. 



Histology of Galls 



Three types of abnormal cell divisions, connected with the formation 

 of galls, may be distinguished, according to the direction that the di- 

 vision takes. In the first type, the regular orientation of the trans- 

 verse partitions cannot be recognized in young galls. In the second 

 type, the cells divide usually in a plane perpendicular to the upper sur- 



