52 BOTANICAL GAZETTE, [ March, 
The development of the different parts of the fibro-vascular 
bundle is most readily followed by means of a series of trans- 
verse sections. Such a section, made Lefore any of the perma- 
nent tissue is formed, shows that the young bundle is composed 
of a mass of thin-walled cells whose diameter is very much less 
than that of the surrounding ground-tissue cells, so that the 
young bundle is clearly defined (figure 
The first indication of the formation of permanent tissue is a 
change in the walls of certain cells arranged in groups at the 
periphery of the cylinder and at equal distances from each other 
(figure 5. ph). Their walls become noticeably thicker and 
strongly refractive, so that they are easily recognized. These 
cells form the beginning of the phloem masses of the complete 
bundle. Shortly after the first phloem cells have become differ- 
entiated, there are formed at equal distances — them, also at 
the outside of the bundle, an equal number of groups, consisting 
at first of two or three cells, satiee: walls become strongly thick- 
ened, but appear — so t they differ markedly from the 
phloem cells. ei are ee primar tracheids and form the be- 
aera ns! the xylem (figure 5. 
itudinal section of a traidle at this stage shows that 
the pe tracheids are narrow pointed cells, with spirally 
thickened walls, the spirals more or less confluent so as to form 
reticulate markings (figure 7. tr). 
The further development of the bundle proceeds very slowly, 
the formation of permanent = continuing from the points at 
the circumference toward the center. e secondary tracheids, 
for there are no true nin are frequently marked with bor- 
dered pits, resembling more those of gymnosperms than the 
sealariform markings of the vessels of ferns. Their formation 
ean be readily followed by making — longitudinal sections at 
points where the bundle is not fully formed. 
These markings begin by the sivintibe line of the wall leaving 
spaces of considerable size where the wall remains of its original 
thickness. At this stage (figu S 2s the wall appears marked 
with large bat shallow pita: the thickening progresses, 
while the bottom of the pit tees its original diameter, the 
parts of the wall surrounding it grow over it so as to make the 
pit assume more and more the form of an inverted cone. The 
upper opening does not close up, but after it has reached a certain 
size its walls cease to approach each other and the upper part 0 
the pit has the form of a short tube, so that the whole pit is like 
an ordinary inverted funnel. On account of the uniform diam- 
eter of the neck of the funnel, its vata, when seen from above, 
