324 Transactions of the Canadian Institute [vol. ix 



elsewhere. But this gall is an exception to the rule. While the outer 

 walls of the epidermal cells are considerably thickened in the normal 

 they are much less so in the gall. Also the two layers of coUenchyma 

 cells underlying the epidermis in the normal stem are absent. 



The increase in size of the stem where the gall is situated is due 

 principally to increased cell division in the cortex, since the epidermis 

 produces only two additional layers. The cells produced in the cortex 

 are larger than the normal, but the most peculiar feature to be noted is 

 the mode in which division has taken place and the relative arrangement 

 of the products of division. The location of this tissue is illustrated in 

 Fig. 23. The method of cell division is clearly the same in this species 

 as in the two Lepidopterous types Stagmatophora ceanothiella and Gnori- 

 tnoschema asterella. The clusters contain from 2 to 6 members produced 

 from a single cell. The dividing walls are straight, and at the stage ex- 

 amined had the greatly elongated nuclei in close contact with them. 

 These nuclei were seldom exactly opposite but usually diagonally across 

 from one another. These characteristics are represented in Fig. 23. 

 Schiirhoff*^ has described the mode of division in callus, contrary 

 to the views in vogue, he states that the nuclei divide mitotically only. 

 There is good reason to believe that the phenomena observed here corre- 

 spond very closely to those given in his account. 



Near the inner edge of the cortex, glands are regularly spaced around 

 the stem. This is a rather remarkable fact as they do not occur normally 

 in this part of the stem. Indeed it was only after a careful search that 

 they were located in the transition region between root and stem. The 

 search was extended to other species with the result that they were 

 found in Eiipatoria purpureum L. in the roots, the cortex and the re- 

 productive axis, but in E. urticcBfolium Reichard as in E. perioliatum L. 

 only at the base of the stem. 



Rhahdophaga batatas Walsh. 



"On Salix humilis Marsh. A polythalamous gall of very variable 

 shape and size, pale green when young, the colour of the bark when mature, 

 growing on twigs .06-. 19 inch in diameter and always some distance from 

 the tip of the twig. Sometimes it resembles a small kidney-potato 

 pierced lengthways by a twig, and has then most generally a smooth 

 polished surface studded with a few buds, one or two of which occasion- 

 ally give birth to a shoot, and it then reaches 1.35 inch in length and 

 .6 inch in diameter. Sometimes it resembles a young apple pierced 

 lengthways by a twig and attains a diameter of .3 inch." — Walsh.** 



In this gall the larval cells are situated in the pith of the host plant, 

 just inside the line of the fibro-vascular bundles. The epidermis has a 

 much thicker cuticle than that borne by a normal stem of corresponding 



