Cedar-Apple (Gy mno sporangium Macropus ). 265 
it composed of the same tissues that go to make up the 
substance of the normal leaf, but all of them modified, 
the differences being specially marked in the epidermal 
system. 
The vascular system enters the knot as one bundle given 
off from the vascular bundle of the branch of the tree. At 
first it is doubtless a simple bundle of the collateral type, but 
as the knot increases in size the vascular bundle developes 
rapidly until it soon appears like the vascular system of a 
branch — a number of collateral bundles placed side by side, 
thus forming a complete zone at the centre of a cross section. 
Soon after entering the base of the knot the vascular bundle 
divides into a number of branches which radiate in all direc- 
tions throughout the substance of the knot, and which in 
turn give off small radiating branches. Fig. 2, which is a 
diagram of a longitudinal section of one of these knots, shows, 
a , the location and manner of branching of the vascular 
bundle. A cross section of one of these small bundles, such 
as would be found at a\ appears as a nearly circular mass 
of somewhat irregular cells surrounded by larger, more nearly 
circular, parenchymatous cells. Comparing this with the cross 
•section of the bundle of a normal leaf, there is found a strong 
resemblance, but the elements of the bundle of the knot are 
enlarged, show a marked indication of distortion, and there 
is little distinction between xylem and phloem. The larger 
part of the vascular bundle of the knot is composed of scalari- 
form and reticulated tracheids. 
Much the larger part of the knot is formed by somewhat 
elongated, parenchymatous cells, closely resembling those 
of the normal leaf, but much larger, with thicker walls, and 
the cells themselves rather more loosely packed together, 
leaving very many and quite large intercellular spaces. The 
position and relative extent of this tissue is shown in the 
diagram, Fig. 2, b. Commonly each cell appears as if it had 
a distinct wall of its own, so that between two adjacent cells 
the wall is double, but sometimes these walls have the ap- 
pearance of having fused together, and often at these places 
