1916] COSENS & SINCLAIR—AERIFEROUS TISSUE 223 
of hair present on the Acarina dimple gall on the leaves of Acer 
Negundo L. are exactly reproduced by the form occurring on the 
reproductive axes, although the normal leaf hair is straight. 
An almost dormant characteristic has also been aroused in Rosa 
blanda Ait. by the cynipid producer Rhodites multispinosus Gillette 
(CosEens 2). The gall produced is always exceedingly spiny, 
although the stem of the host from which it originates is usually 
unarmed. But it is worthy of note that the production of spines 
is a marked character of other species of the genus. Doubtless 
further investigations will show that examples of this kind can be 
almost indefinitely multiplied. 
The ‘reinstatement in a gall of vestigial characteristics of the 
plant has an important bearing on the question of gall formation. 
The producer has long been recognized as exercising a directive 
control over the activities of the protoplasm of the host, but these 
examples of the rehabilitation of dormant characters show that the 
forces operative in gall formation are of wider scope. Under these 
conditions unexpected structures and unusual combinations may 
well be produced, and in the interpretation of the morphology of 
any gall it becomes necessary to discriminate carefully between 
these two classes of organs and tissues, that is, between those that 
are simply environmental modifications of the normal and those 
that are vestigial or in use in other parts only of the plant. Inci- 
dentally it may be added that there remains no authentic instance 
of any organ or tissue in a gall that is new, ontogenetically or phylo- 
genetically, to the host. 
_ This investigation was carried on at the Botanical Laboratories 
of the University of Toronto, and the authors wish to acknowl- 
edge their indebtedness to Professor J. H. Favutt for invaluable 
criticism throughout the whole course of the work. 
UNIVERSITY OF TORONTO 
LITERATURE CITED 
1. Burns, Geo. P., Heterophylly in Proserpinaca .palustris. Ann. Botany 
18: 579-587. pl. 38. 1904. 
2. Cosens, A., A contribution to the morphology and biology of insect galls. 
Trans. Canadian Inst. 9: 297-387. pls. 13. 1912 
