332 BOTANICAL GAZETTE [SEPTEMBER 
that they may persist in the reproductive axes, petioles, leaf or 
branch traces, and roots. The tissue under consideration must be 
regarded as primitive in accordance with this general principle. 
It occurs in the pith and cortex of the reproductive axes of all species 
of Populus and Salix. It is found in the pith at the bases of the 
branches of the stem of Populus and Salix, and only in that region 
in the case of the latter genus. Further evidence in support of the 
same view is furnished by its presence in the branch gaps, its occur- 
rence in petioles and the vicinity of stem and leaf traces, and in the 
cortex of the root. This prominent development of the tissue in the 
vestigial carriers of both Populus and Salix, taken in conjunction 
with our experimental results, presents strong evidence in favor of 
the theory that the aeriferous tissue in the Salicales is of ancestral 
type. | 
The power of the gall stimulus to arouse latent properties in the 
protoplasm of the host, expressed by its ability to produce an 
aeriferous tissue not normally present in Salix, is also exemplified 
in the case of glandular tissue and trichomes in certain galls on other 
plants. It may be stated as a general principle that glands, when 
they are present normally in a tissue of the host, are always more 
plentiful or larger in the gall originating from that tissue. A strik- 
ing example (CosEns 2) of this is furnished by the pithy spherical 
stem gall produced on Solidago canadensis L. by Eurosta solidaginis 
Fitch. But in some cases glands are produced in galls on parts of 
the host normally glandless. Thus, they are plentiful in the stem 
gall Neolasioptera perfoliata Felt on Eupatorium perfoliatum L., but 
are not present in the same location in the normal plant. In fact, 
they were found in normal plants in the transitional region between 
stem and root only, both in this species and in E. urticaefolium 
Reichard. 
Trichomes exemplify the same principle. When the gall pro- 
duces unusual trichomes, the abnormal forms can almost invariably 
be found on the reproductive axes of the host. Thus, while the 
normal hairs on the leaf of Quercus macrocarpa Michx. are of the 
stellate type, those of the reproductive axes are acicular, correspond- 
ing exactly to those composing the pubescence of the leaf gall 
Eriophyes querci Garman on this host. Also, the convoluted type 
