STERLING: KRAMERIA CANESCENS GRAY. 367 
pits. Next to the medulla, and formed in the protoxylem, lie 
the small spiral tubes, while adjacent to them are the some- 
what larger reticulate tubes (fig. 9, plate III). But through- 
out all of the rest of the xylem only tubes with bordered pits 
are to be found. They have lignified walls 1.5 to 3 microns in 
thickness, and average about 30 microns in diameter. Although 
they occupy a much smaller area of the xylem than the tra- 
cheids, the water tubes have relatively much greater water- 
carrying capacity, the total, for the stem given above, being 
14.5 per cent of the xylem or 6 per cent of the whole stem. 
The tissues of the xylem are not distributed in such a man- 
ner as accurately to indicate the periods of growth. However, 
as the larger water tubes are in rather loosely formed rows, 
concentrically arranged, and frequently accompanied by rows 
of wood parenchyma, they appear to form the boundary lines 
of the periods of growth. The wood parenchyma cells are de- 
veloped prior to the larger water tubes, thus indicating that the 
wood parenchyma cells are formed at the end of one growing 
season and the larger water tubes at the beginning of the 
season following. The vertically elongated cells contain many 
circular straight pits (figs. 22 and 23, plate V) ; and as they are 
abundantly supplied with starch, they furnish additional evi- 
dence that the wood parenchyma is formed at the end of the 
season’s growth for the storage of reserve materials. 
Separating the vascular bundles are the numerous narrow 
medullary rays, usually consisting of a single row of cells. In 
the xylem portion the walls are relatively thin, lignified in the 
older part only, and contain numerous straight, circular pits. 
They are from three to six cells in height, and vertically elon- 
gated (figs. 20 and 21, plate V). Frequently the tracheids 
crowd in upon the medullary ray cells and cause a thickening 
of their tangential walls (fig. 5, plate Il). The cells of the 
phloém portions have cellulose walls, and as the outer ones are 
tangentially stretched the rays become wedge-shaped. In all 
parts the cells are well supplied with starch—a fact which, 
taken in connection with their structure, indicates that their 
chief function is that of storage. 
At the center of the stem, surrounded by the vascular 
bundles, is a rather large medulla. The cells have lignified, 
pitted walls. Those composing the central part are nearly 
isodiametric (fig. 25, plate V), while the marginal cells are 
somewhat narrower and vertically elongated (fig. 9, plate II). 
