358 BOTANICAL GAZETTE [NOVEMBER 
finally entering the leaf base. There is such a girdle for every leaf, 
and every girdle must cut, on account of its course and location, every 
other like girdle in two places. Every girdle receives on its inner 
edge branches from the central vascular bundles which leave the 
vascular cylinder at various places, and sends out branches from its 
outer edge to other girdles. From this vague conception, which does 
not at all agree with the drawings of METTENTUS, the current text- 
book accounts have been drawn. These accounts, however, do not 
really interpret METTENIUs, but are as far from his interpretation as 
that was from the true situation. 
In his description of the structure of the bundles METTENIUS was 
more fortunate, and made a very important contribution. He says 
that those bundles which are to leave the vascular cylinder are marked 
off by broad medullary rays and are more definitely bounded than 
those which continue in the cylinder; that on the inner edge they are 
provided with spiral vessels (protoxylem), while the others are Pro — 
vided with reticulated cells in the same relative position. At the region 
of the outward bend of the trace the vascular elements are grouped 
with the spiral elements on the inner or upper side, immediately 
bordering the reticulated elements beneath; and this structure the 
girdle retains in encompassing the stem. Before entering the leat, 
however, a change in structure begins to occur, and is completed in 
the lower part of the petiole; after which the bundle remains un- 
changed up to the pinnae. The first indication of this change the 
appearance of thin-walled cells in the vicinity of the spiral (pr ey 
lem) elements, separating them from the reticulate elements. 8 
the further course of the bundle the spiral (protoxylem) seit 
gradually move farther within, and the wood is now divided into ui 
parts by the thin-walled cells; the inner part developing its elemen = 
constituents centripetally, and the outer part centrifugally. #1 
the spiral (protoxylem) elements are found in the outer part be 
bundle, and the centrifugal part is still more reduced, while 
centripetal part has reached its maximum development. This oe 
ture of the bundle is retained in its further course in the petiole = 
in the pinnae, where in Dioon edule the centrifugal wood is a 
altogether. Such is a very brief statement of the descriptio? i os 
transition from centrifugal to centripetal xylem in the leaf tac 
the 
the 
