27G The Ohio Journal of Science [Vol. XVI, No. 7, 



eventually drops its larval chamber enclosed by the nutritive 

 and protective layers. This central part of the gall which 

 slips out has the shape of a short, blunt horn. The tapering 

 of this structure toward the proximal part of the gall makes 

 possible the easy departure of this larva containing portion 

 when the dehiscence layer surrounding it gives way. The gall 

 aggregates commonly occur at or near the end of the stem, the 

 tissue, after the larval chambers have fallen, dying, turning 

 black, giving the twig an unsightly appearance (PI. XIX, Fig. 6). 

 In the case of a single gall (PI. XIX, Fig. 4) the stem is not 

 killed, but the tissue of the "socket" part is cut off by an 

 abscission layer (Fig. 16a, PI. XVIII). 



The gall on the fruit (PI. XIX, Fig. 8) possesses exactly 

 the same structure as those on other parts of the plant. The 

 galls shown are not mature, the chamber not having burst 

 through the surrounding supporting tissue. In section (PI. XIX 

 Fig. 4) the galls are seen to project into the ovulary cavity, 

 exhibiting in their entirety the characteristic shape observed 

 in those of the leaf which project from both sides of the leaf. 

 The ovule is aborted. 



The most important histological feature is naturally 

 associated with the chief feature of the gall and consists of 

 the dehiscence layer developed around the protective layer. 

 This layer (PI. XVIII, Fig. 16a) is made up of extremely thin 

 walled cells, arranged in rows, radiating from the protective 

 layer. It gives evidence of having been formed by rapid 

 division when the gall was nearing maturity and becomes 

 intercalated between the fibro-vascular system and the pro- 

 tective layer. Its eventual disintegration separates it cleanly 

 from the protective layer, leaving the central part containing 

 the larva free to be shaken out by the wind. 



A few similar types of galls are known among the Cynipidas 

 and Itonidid^. Houard (11) figures an itonid (Oligotrophus 

 Reaumurianus Loew) which is exactly similar. It occurs on 

 Tilia grandifolia of Europe. 



Comparative Studies. 

 Kataplasmas. 



The two kataplasmas (galls 1 and 2) possess differences 

 related in part to the position of the parasites on the stem. The 

 excessive hyperplasia of the cortex, in the case of the witches- 

 broom branch bases, seems to be associated with the superficial 



