1912] Morphology and Biology of Insect Galls 355 



Even in one genus there may be considerable variation in the degree 

 of development of the protective zone. It is entirely absent in Rhodites 

 gracilis Ashm. and R. bicolor Harr., but two distinct layers are found in 

 R. ignotus O.S. 



In several species of the genus Andricus canals were found passing 

 from the exterior towards the larval chambers. In the early develop- 

 mental stages these opened into the gall cavity, but later were blocked by 

 outgrowths of sclerenchyma from the protective zone. They were located 

 in the species Andricus piger Bassett, A. petiolicola Bassett and Andricus 

 N.S. (Figs 43-47)- 



A canal similar to those in the Andricus genus was found also in 

 Dryophanta palustris O.S. In this species the plug of sclerenchyma is 

 not developed (Fig. 49). 



An epidermal scale was found in the bottom of a depression at the 

 apex of the galls produced by certain species of Rhodites. Below each 

 scale a small mass of sclerenchyma is differentiated. These structures 

 seem to be homologous to the canals in the genus Andricus. They are 

 present in the following species: Rhodites ignotus O.S., R. gracilis Ashm. 

 and R. nebulosus Bassett. 



The gall Solenozopheria vaccinii Ashmead has the sclerified tangen- 

 tial walls of its protective zone much thicker on one side than on the 

 opposite. This is very unusual in stem galls, although a common feature 

 in leaf galls. 



The collapsing of the cells of the nutritive zone after the withdrawal 

 of the contents is exemplified in almost any gall studied. It is, however, 

 particularly noticeable in Dryophanta palustris O.S. (Fig. 52). 



Empty cells were found throughout the nutritive zones in the later 

 stages of nearly all the galls examined. Good examples of this pheno- 

 menon are furnished by Andricus singularis Bassett and Aylax glechomcs 

 Linne. 



The separation of the tissues so as to produce a free larval chamber 

 gall is shown in the species Holcaspis globulus Fitch and H. bassetti 

 Gillette; also in Dryophanta palustris O.S. (Fig. 50). In the last species, 

 as Cook^ has shown, the separation of the larval chamber takes place at 

 a very early developmental stage. 



Mitotic phenomena were observed in the cambium and near it in the 

 adjoining parenchyma zone of Dryophanta palustris O.S. The number 

 of chromosomes remains as in the normal. Good examples of amitosis 

 were located in the nutritive tissues of Aylax glechomcB Linn6, and 

 Aulacidea nabali Brodie (Text Fig. 6). 



The parenchyma zone of Amphibolips confluens Harris furnishes an 

 example of a tissue consisting of long filamentous cells from which 



