1912] Morphology and Biology of Insect Galls 333 



gall. The abnormal cells from the palisade parenchyma show clearly 

 their origin by their arrangement in rows at right angles to the surface 

 of the leaf. The cells produced by the spongy parenchyma, on the 

 other hand, are not regularly placed but include air spaces. The result 

 is that the abnormal tissue in this case also resembles the normal tissue 

 from which it is derived. 



This stage of the gall shows that the cavity present in the mature 

 gall has arisen between the tissue produced by the spongy and that 

 derived from the palisade parenchyma of the normal leaf. 

 Pontania hyalina Norton. 



"Fleshy galls occurring in two parallel rows, one on either side of the 

 midrib, sometimes touching but not originating from the latter, and 

 rarely extending to the edge of the leaf; sometimes as many as twenty 

 on a single leaf; in other cases confined to a row on one side of the leaf 

 or occasionally occurring singly; shape irregular elongate-ovate, pro- 

 jecting equally on both surfaces of the leaf; length 7 to 10 mm.; the 

 abortive ones smaller. Colour on upper side more or less brownish red ; 

 beneath white, with slight purplish tinge." — Marlatt.^^ 



The anatomy of this gall presents scarcely any differentiation of 

 tissue. When mature it consists of a mass of thin-walled chlorophyll- 

 bearing cells, the innermost of which are arranged in rows almost at 

 right angles to the blade of the leaf, as seen in Fig. 75. Cells are so much 

 alike that they afford no clue as to their origin. 



Again I was able to obtain material so young that the larva was still 

 confined within the egg membrane (Figs. 73, 74). It shows that the 

 spongy parenchyma, the palisade parenchyma and the epidermis of the 

 normal leaf were jointly concerned in the production of the abnormal 

 tissue. The spongy parenchyma has contributed nearly half of the 

 entire mass, the epidermes three layers each, and the row of cells 

 immediately overlying the lower epidermis three layers. The remainder 

 has been derived from the palisade parenchyma. 



Undescribed Sawfly Gall (Pontania N.S.) on Salix humilis Marsh. 



Fig. 3. — ^Two galls attached on opposite sides ol the midrib. 



