THE SEEDLING AND YOUNG PLANT 63 
but other arrangements occur. In the parts where 
fewer vessels occur it is not uncommon to find a series 
of radial rows of about six to ten fibres end in a single 
parenchyma cell, and thus are formed short, tangential 
rows of wood-parenchyma cells, intercalated, as it were, 
between the radial rows of other elements (fig. 12, p). 
It often happens, moreover, that reticulated and pitted 
vessels are closely surrounded by wood-parenchyma. 
The secondary medullary rays exist as single radial 
rows of cells, agreeing in form, &c., with the cells 
of the primary medullary rays. In contact with one 
another or with wood parenchyma their walls have simple 
pits, but they have bordered pits where they abut on 
tracheids or vessels. In winter these cells are filled with 
starch. On tangential sections (fig. 15) it is easy to 
see how the vertical groups of cells have the same origin 
as the groups of wood-parenchyma cells—the difference 
being that the cambial cells which are going to be trans- 
formed by horizontal divisions, &c., into vertical rows of 
ray parenchyma, undergo repeated tangential longitndi- 
nal divisions, and so continued radial rows are formed. 
The cells of these rays are often much shorter than 
those of the wood-parenchyma, yet all gradations occur. 
The mother-cells may be very long, evidently corre- 
sponding to two, and they may also divide in the radial 
longitudinal plane, and the ray become biseriate. 
These secondary rays start (on the transverse section) 
from the first large vessels, or from younger ones, or 
they may start from other points. The ray may some- 
