360 Transactions of the Canadian Institute [vol. ix 



which the stimulus is applied in one direction only. The stem mothers 

 of the genus Chermes thus become surrounded by a ring of tissue that 

 grows out around the point of attachment of the insect (Fig. 11). The 

 Dipterous gall Cecidomyia ocellaris O.S. also furnishes a very striking 

 example of this phenomenon. In this species the leaf is scarcely at all 

 thickened under the larva, but the proliferation is so marked around it 

 that the producer ultimately lies in a concavity, not formed by the leaf 

 becoming depressed, but by the outgrowth of the circular ridge of tissue 

 (Fig. 33). Any explanations offered to account for these facts are merely 

 conjectural, but it seems likely that the enzyme content requires a certain 

 degree of concentration in order to exhibit its maximum activity, and 

 that immediately in contact with the larva it has not the requisite dilution 

 to cause the greatest proliferation. It is a well-known fact that the 

 amount of growth of plants in culture solutions varies with the degree of 

 concentration of the nutrient substance in the medium ; thus there is an 

 optimum quantity and as this is exceeded growth is more and more 

 inhibited. An example of this is furnished by the checking of the growth 

 of Penicillium when the culture solutions are too concentrated. 



With regard to the question, "What becomes of the epidermis under 

 the egg?" I agree with Weidel that there is little likelihood of abnormal 

 cell production until the larva punctures the egg membrane, but when 

 this occurs the epidermis becomes part of a nutritive zone and will 

 undergo such rapid changes that its epidermal characteristics will soon 

 disappear. The chief alterations will be expressed in the much richer 

 contents of the cells and in their steady collapsing as these contents are 

 withdrawn (Figs. 49, 51, 52). The latter change makes it extremely 

 difficult to follow the normal into the abnormal epidermis unless at an 

 extremely early stage. While the enclosing of the larva is due chiefly to 

 the growth of the surrounding tissues, yet the collapsing of the nutritive 

 layer will assist it to a certain extent. 



Weidel's photographs show that in Neuroterus there is not at any 

 stage an opening into the larval cavity that is lined with the epidermis of 

 the leaf, and that after the larva enters its prepared chamber the opening 

 is very soon closed. In the method of development as stated by Beye- 

 rinck we would expect to find such an opening persisting for some time, 

 and if we do, that must be accepted as confirmatory evidence of the 

 truth of his hypothesis. In two different genera, namely Dryophanta 

 and Andricus, I have found canals leading into the gall cavity in the 

 early developmental stages (Figs. 43-49). The epidermis of these struc- 

 tures is continuous with the gall epidermis and it bears the same class of 

 trichomes as the latter. The canal is very marked in Dryophanta palus- 

 iris O.S., and its lining which is the same as the gall epidermis, can be 



