1912] MORPHOLOGY AND BlOLOGY OF INSECT GaLLS 33 1 



produced from each. The epidermis bears numerous lenticels, organs 

 which Kiister'^ mentions as occurring on the gall produced by Pontania 

 salicis. 



At the point of attachment of the gall the blade of the leaf is strength- 

 ened by several rows of cells derived from the upper epidermis and the 

 palisade parenchyma, as shown in Fig. 81. These cells seem to have 

 remained unmodified in any way, since their arrangement in rows is still 

 clear in fairly old stages of the gall. Consequently they differ very 

 markedly from the irregularly arranged cells of the main part of the gall 

 body. 



Pontania pomum Walsh. 



"On Salix cordata (and very rarely on S. discolor). A smooth, 

 fleshy, sessile, globular or slightly oval, monothalamous gall, resembling 

 a miniature apple, .30 to .55 inch in diameter, growing on one side of the 

 midrib of a leaf, and extending to its edge or sometimes a little beyond it. 

 The principal part of the gall generally projects from the under side of 

 the leaf, and only about one-sixth of its volume from the upper side, 

 although very rarely it is almost equally bisected by the plane of the leaf. 

 Scarcely ever more than one gall on a leaf and very rarely two of them, 

 more or less confluent so as to seem like one kidney-shaped gall. Ex- 

 ternal colour greenish-yellow, generally with a rosy cheek like an apple 

 especially on the upper surface and often with many dark little dots on 

 its surface." — Walsh.** 



The ovipositor of the producer of this gall has been thrust laterally 

 through the midrib of the leaf into the mesophyll. The wound has com- 

 pletely severed the bundle of the midrib, as seen in Fig. 76. 



The full-grown gall presents an epidermis with a very thick cuticle. 

 The remainder of the gall consists of a complex of thin-walled cells 

 arranged so as to constitute a typical aeriferous tissue (Fig. 77). A 

 similar arrangement of cells is not found in the normal leaf, the mesophyll 

 of which consists of a fairly compact tissue. The vascular strands growing 

 out from the wounded bundle form a complete ring around the gall, 

 situated about half way between the epidermis and the centre. 



I was successful in obtaining this gall at such an early stage that 

 the egg membrane was still unbroken (Fig. 76). This phase shows that 

 the epidermis, the palisade and the spongy parenchyma mutually take 

 part in the gall production. Counting along a line passing through the 

 centre, four of the cell layers are seen to have arisen from the lower 

 epidermis and six from the upper, eight from the palisade and fifteen 

 from the spongy parenchyma of the leaf. Hence it is noteworthy that 

 the new tissues are not the product of a cambium but have been contri- 

 buted to by every morphological region of the leaf. The cells that are 



