326 Transactions of the Canadian Institute [vol. ix 



The cells that compose the principal mass of this gall are slightly 

 smaller than those of the normal pith but in other respects they resemble 

 them very closely. The irregularity in position of the fibrovascular 

 bundles and their imperfect development are well marked features. 

 Yet a sufficient water supply is ensured to the tissues by vascular strands 

 that are given off from the bundles. These strands extend in a radial 

 direction towards the centre of the gall. 



The cortex is considerably thicker than that present in the normal 

 stem. This is due in part to the greater number of cell layers, but also 

 to an increased size. 



The glands that are present in the normal stem of Solidago cana- 

 densis L. occupy certain fixed positions. One gland is present in the 

 cortex outside each bundle and one inside in the region of the pith. The 

 glands found in the cortex of the gall are very much enlarged and have 

 not their characteristically regular arrangement. In the gall pith they 

 are abundant throughout (Fig. 42), but decidedly more plentiful in the 

 vicinity of the fibro-vascular strands. This is the most striking example 

 of gland proliferation found in the galls studied. 



The tissues that supply the larva with food are not differentiated 

 into a nutritive zone. 



Summary. 



The galls produced by this order of insects vary very much in their 

 degree of complexity. Some forms such as Cecidomyia ocellaris O.S. are 

 as simple in structure as an Acarina, "Dimple", gall; other species as 

 Rhabdophaga batatas Walsh present all the specialized anatomical char- 

 acteristics of a Cynipid gall. 



The abundant production of glands in tissue under stimulation is 

 very clearly exemplified in Eurosta solidaginis Fitch. At first sight it 

 appeared as if glands were not present in the host of Neolasioptera per- 

 foliata Felt, but they were located at the base of the stem and in other 

 species of Eupatoria. 



The unequal thickening of the tangential walls of the sclerenchyma 

 protective layer in Rhabdophaga batatas Walsh is a very unusual pheno- 

 menon in this group. 



Cecidomyia triticoides O.S. is the only gall of this group in which a 

 well defined crystal layer was found. In it each cell lumen is entirely 

 filled with a single crystal of calcium oxalate. 



The production of the aeriferous tissue, that occupies practically 

 the entire pith in the gall Cecidomyia triticoides Walsh, is one of the 

 most interesting phenomena exhibited in this group. The nature of this 

 will be discussed in the biological section of the paper. 



The collapsing of the nutritive zone after the cell contents are with- 

 drawn is well exemplified in Cecidomyia triticoides Walsh (Fig. 37)- 



