332 Transactions of the Canadian Institute [vol. ix 



produced at this stage are in rows generally in exact alignment with the 

 cells from which they have arisen. They thus do not have the arrange- 

 ment of the aeriferous tissue of later stages to which they give rise. 

 The cuticle, so marked a feature of the older stages of the epidermis, is 

 exceedingly thin. The epidermis bears trichomes springing from the 

 bottoms of deep pits (Fig. 76). This condition has arisen through the 

 circumstance that the primary epidermal cells from which hairs have 

 grown out have not experienced the periclinal divisions participated in 

 by their fellows and so have been left far below the general surface as 

 shown in the text fig. below. 



Fig. 2. — Hairs originating from pits In the epidermis of P. pomum Walsh. 



Pontania desmodioides Walsh. 



"On Salix humilis. A smooth, flattish, fleshy, sessile, yellowish- 

 green monothalamous gall of a semicircular outline, the chord of the 

 semicircle adjoining the midrib of a leaf; its general shape like the seed 

 of a Desmodium or like the so-called "quarter" of an orange, the thin 

 inside edge of the "quarter" closely hugging the midrib of the leaf, and 

 the robust outer surface not biangulated but rounded off. No rosy cheek. 

 The volume of the gall is generally about equally divided between the 

 upper and lower sides of the leaf but sometimes the lower portion is 

 rather the larger. Usually there is but a single gall on a single leaf, but 

 occasionally there are two of them, either on the same side or on opposite 

 sides of the midrib." — Length .23 to .50 inch Walsh.*^ 



When mature this gall shows in cross section a cavity surrounded 

 by a peripheral layer of little differentiated tissue. The epidermis has 

 given rise to a very thick cuticle that is not present in the normal leaf. 

 The bundle of the midrib has been injured only slightly by the ovipositor 

 of the producer. The vascular strands given off from it almost encircle 

 the gall along a line half way in from the epidermis. 



A stage of the gall so young that the larva was unhatched shows 

 the gall tissue to have been produced by cell division in the upper epi- 

 dermis, the spongy parenchyma and the palisade parenchyma of the 

 normal leaf (Fig. 80). At the thickest part of the gall, when it is in this 

 stage, the upper epidermis has produced four layers of cells, the spongy 

 and palisade parenchyma seven layers each. The lower epidermis has 

 not divided as yet, and probably takes no part in the production of the 



