Robinson and Walkden.— A Critical Study of Crown Gall.. 303 
a temperate greenhouse. For inoculation and as controls we used shoots as 
nearly as possible similar and equivalent. Where the lower end of a shoot 
was employed, this was severed from the plant by a clean transverse cut 
across the middle of an internode, and then, either with or without inocula¬ 
tion, the shoot was planted as a cutting in moist soil or sand. In other 
cases the apices of vigorous shoots were removed by transverse cuts through 
a definite internode as near as possible to the apex, and the surfaces so 
produced were either inoculated or left as controls, the plants being sub¬ 
sequently grown under ordinary greenhouse conditions. 
It was found to be immaterial whether the equivalent shoots used as 
controls were on the same plants as those inoculated or on different plants 
of the same age, since precisely similar results were obtained in either case. 
In most of the work, therefore, the control uninoculated surfaces were made 
on the plant bearing the inoculated shoots. At suitable intervals of time, 
usually three days in the early stages, small pieces of stem, including either 
the inoculated or the control surface, were fixed in weak chrom-acetic acid 
and embedded and microtomed. Sections were usually stained with iron- 
alum-haematoxylin, followed by orange G in clove oil, or by carbol fuchsin 
and orange G. In addition, for special purposes hand-sections were 
examined in water in the fresh condition. 
Normal Anatomy of the Shoot. 
PI. V, Fig. 5, is a transverse section of the stem in the region j ust below the 
apex, where the inoculations were usually made for the production of aerial 
galls. The stem is of the ordinary dicotyledonous type. There is a distinct 
hypodermis and a small amount of collenchyma is disposed in patches. 
The cortical parenchyma extends about six layers of cells deep and is 
limited by a definite starch sheath. Mucilage ducts develop at regular 
intervals in the cortex near to the starch sheath, being usually situated 
opposite the primary medullary rays between the larger vascular bundles of 
the ring. The vascular ring consists of larger bundles with smaller ones 
between, and even at this early stage of development the pericycle is well 
defined opposite the vascular bundles as fibrous cells with distinctly 
thickened walls of pure cellulose. At this stage these pericycle cells 
retain their nuclei and living contents. The phloem of the bundles is 
a narrower zone within the pericycle and is separated from the xylem by 
the thin-walled desmogen cells which later function as cambium. The 
xylem consists of five or six radial rows of metaxylem and protoxylem 
vessels with conjunctive parenchyma and xylem parenchyma in the 
protoxylem region. The perimedullary zone is parenchymatous, but is.not 
very clearly differentiated from the parenchymatous pith. The medullary 
rays are relatively narrow, being at most only eight cells wide. 
X 
