Pe NON el eae, Cb 
366 Structural and Physiological Botany. © 
any strongly developed spiral thickening, nor usually even 
the spiral striation already mentioned (p. 16). Inthe winter, 
especially when vegetation is dormant, they usually contain 
starch. It is comparatively rare for the wood-cells to 
become septated, when their thickening-layers are nearly 
completely formed, by one, still more rare by several parti- 
tion-walls, into daughter-cells, which are then enclosed by the 
thick-walled wood-cells, and are termed septated wood-cells. 
They rarely possess pits ; and, when present, they are cleft- 
shaped, and placed obliquely in a spiral running to the left. 
In the winter they usually contain a small quantity of starch. 
The parenchymatous cells of the xylem, which are scarcely 
ever wanting in woody Dicotyledons, are distinguished from 
the other elements of the vascular bundles by their walls 
being less strongly thickened; by their having pits which 
are always closed and never bordered, and by the absence 
of any spiral thickening. They may be formed in three 
different ways :—-by direct growth from a cambium-cell, or as 
a daughter-cell either in a cambium- or in a young prosen- 
chymatous cell. In the first case they are isolated ; in the 
two latter cases they are still enclosed within the walls 
of their parent-cells. These cells also usually contain starch 
in winter. : 
The vessels of the xylem have usually bordered pite. 
In the early condition of the vascular bundles, annular, 
spiral, and reticulated vessels are found only near the pith, 
in the medullary sheath ; the younger parts, on the con- 
trary, contain nothing but pitted vessels. ‘The partition- 
_ walls of the separate vascular cells are either horizontal or 
oblique, and scalariform or penetrated by pits, ze by a 
round orifice. A scalariform perforation has at present only 
been observed in pitted and reticulated, but not in spiral or 
annular vessels. When the walls of the vessels are not 
spirally thickened, the pits are most numerous on the side 
where the vessel borders other vessels or thin-walled wood- 
~ cells. At these spots the borders of the pits are also the 
