1912] Morphology and Biology of Insect Galls 315 



As in the preceding galls described, the principal part of the gall 

 tissues in this species is originated from the vascular bundles and paren- 

 chyma strands (Fig. 17). The latter are very wide and the abnormal 

 cell division is more marked in them than in the bundles. The wood 

 elements produced remain undifferentiated and pith-like. The cuticle 

 of the gall epidermis is much stronger than that found in the normal stem. 

 The epidermis itself has responded to the stimulation by the production 

 of an exira layer of cells. The cortex of the gall contains approximately 

 one-third more cell layers than the normal cortex as seen in Fig. 18. 



The normal stem of Ceanothus americanus L. contains glands in 

 the cortex. These are fairly regularly spaced around the stem but are 

 larger and more numerous at the nodes. Glands occur also in the pith 

 of the stem, the petioles of the leaves and the reproductive axes. But in 

 parts of stems, contiguous with galls, though glands occur in the pith there 

 are none in the cortex, except at the nodes. Glandular cells, however, 

 are plentiful in the cortex of such stems. 



A cross section of a gall shows larger and more numerous glands 

 than a corresponding section of the normal stem. The probable ex- 

 planation of this is that owing to the shortening of the stem axis, nodes 

 are cut more frequently. In the gall cortex there are also narrow, elong- 

 ated, glandular cavities, that do not seemingly correspond to anything 

 seen in the normal stem. They require further elucidation. They are 

 in groups each containing three or four glands, as illustrated in Fig. 19. 



Summary. 



The galls are all of a comparatively simple type, for while there 

 is considerable proliferation in the tissues there is little diiTerentiation. 

 The medullary rays and vascular bundles respond the most readily to 

 the gall stimulus, yet cell division takes place in the epidermis of the 

 species Stagmatophora ceanothiella Cosens. 



The highly specialized habits of the larva, developed in caring for 

 the welfare of the imago, make the group very interesting. Thus in 

 each of the forms studied provision is made by the larva for the emergence 

 of the moth from the gall. These habits are seen at different stages of 

 development. In Stagmatophora ceanothiella Cosens and Eucosma 

 scudderiana Clemens the gall wall is simply gnawn partly through, while 

 in the Gnorimoschema genus an aperture of exit is carefully prepared 

 and plugged. These different methods of procedure are remarkably 

 suited to the habits of the insects. In the former a plugged exit would 

 not be suitable as the insect winters in the larval condition and the drying 

 of the gall would prevent the plug from slipping out easily. In the latter 

 the galls are still green when the insect becomes mature and the plug 

 mechanism is preferable. It is clear then that in these galls the producer 



