. Biological series. Biology. 334 Transactions of the Canadian Institute [vol. ix This is a monothalamous gall found on the leaves of Salix humilis. It is spherical in shape and in that feature resembles P. pomum, but in other respects it differs very markedly. It extends from the side of the midrib almost out to the margin of the leaf and is divided into two hemispheres by the leaf blade. In consequence the gall protrudes nearly equally from each leaf surface. Usually there are two or three galls on a leaf. When two are present they come in contact with the mid- rib at the same place but on o
. Biological series. Biology. 334 Transactions of the Canadian Institute [vol. ix This is a monothalamous gall found on the leaves of Salix humilis. It is spherical in shape and in that feature resembles P. pomum, but in other respects it differs very markedly. It extends from the side of the midrib almost out to the margin of the leaf and is divided into two hemispheres by the leaf blade. In consequence the gall protrudes nearly equally from each leaf surface. Usually there are two or three galls on a leaf. When two are present they come in contact with the mid- rib at the same place but on opposite sides, as illustrated in Fig. 79. In a few cases four and even five galls were found on one leaf. The galls are pubescent but not as densely as the under surface of the leaf. Dimensions:—Average diameter i cm. The mesophyll of the leaf and the upper epidermis are mutually concerned in the production of this gall. In one, sufficiently immature to show the relative amount of tissue arising from each source, it was found that the upper epidermis had produced two cell layers, while the lower had not responded to stimulation; and that the palisade and spongy parenchyma had each produced one-half of the remaining mass. The hollow in the gall, present from the earliest stages, has been formed between the tissue arising from the palisade and the spongy parenchyma respectively. When only one gall originates from the midrib at any point, the vascular bundle is cut approximately half through (Fig. 78). But more frequently two galls are located opposite one another, one on each side of the midrib, in which case the two incisions meet and completely sever the bundle, as seen in Fig. 79. Vascular strands pass almost completely around the gall, along a line half way between the epidermis and the gall cavity. These strands originate from the midrib in the neighborhood of the injury and pass in opposite directions. Undescribed Sawfly Gall on S. lucida Fig. 4.—Two galls produc
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