372 BOTANICAL GAZETTE [NOVEMBER 
sections of a maturer leaf of a seedling clearly shows the transition 
from endarch, through mesarch, to exarch, and shows that it is quite 
independent of secondary wood. The transition begins at the point 
where the strand leaves the central cylinder, and in the seedling pro- 
gresses uniformly and is completed in the petiole; but in the old plant 
the larger proportion of the transition is completed in the lower part 
of the petiole, where only a few strands of secondary xylem remain 
and continue uniform until the bundles enter the pinna, when the 
transition is completed (fig. 25). 
The cause of girdling—No particular cause has been assigned for 
this phenomenon. If such a leaf primordium as is represented in 
figs. 3, L,, and 8, L, be selected, it is possible by careful staining to 
detect four strands which are ultimately developed into vascular 
bundles. The outer (abaxial) ones pursue a more or less direct course, 
but in following them from the stem up, their course at first, after 
leaving the procambium, is vertical; but in the base of the primordium 
they turn inward to a considerable extent (fig. 3, L,). The two innet 
or adaxial strands after leaving the procambial cone pursue quite a 
vertical course, but on reaching a region at the level of the base of the 
primordium, they begin to turn toward it, one on each side, in some 
cases ascending rather obliquely, but generally horizontal from the 
start. Figs. 34, 35, ligt shows the girdling of quite a young primor 
dium, the youngest one in that specimen. It is plain, therefore. ” 
the girdle is established very early in the development of the leaf to 
which it belongs. 
Tracheae.—In examining older plants, the oldest one being ght 
or four years old, a singular phenomenon is noticed. eines 
com- 
various bundles of the stem, which at this time do not yet make a 
plete ring, vertical connections are found, consisting of i 
reticulated elements, branching and anastomosing, but forming a 
tinuous vessels. In the upper part of the plant, where they are 4° 
oping, long and narrow cells are found winding and crowding t a 
between the parenchymatous cells. A little farther down ” smi 
parts of the stem, these cells are found to be multinucleate, with os 
here and there an ill-defined cross-wall. A little farther dows sual 
nuclei disappear, and soon lignification appears, developing 
into well-defined reticulated tracheae. ‘These wind in a very t 
b] 
