Special M orphology and C lassification. 363 
There may be several reasons for this sharp definition of the separate 
annual rings. It is possible that the cells formed in the spring, those 
of the spring-wood, are larger than those of the autumn-wood (see Fig. 
479, p. 373); or their walls. may be thinner, or the cells otherwise of 
a different kind; and the abundance and size of the vessels in the spring- 
-wood must also be taken into account. The structure of the root cor- 
responds to that of the stem in the presence or absence of annual rings. 
The separate annual rings are not always of a uniform thickness, but 
are often more: strongly developed on. one side than on the other. 
From these facts it is possible to form an idea of the life of the tree 
during each year of its growth. Its age is estimated from the number 
of these rings ; from their breadth it is known what years were favour- 
able, and what unfavourable to its growth ; and some knowledge can 
even be attained respecting the growth of the roots, since the growth of 
any side of the stem is known to correspond nearly to the development 
of the roots that branch out on that side. 
Sometimes even in the earliest, but always in the later years, 
the vascular bundles become penetrated in a fanlike manner 
by new parenchymatous cells formed inside themand arranged 
in bands, the secondary medullary rays. Those cambium- 
cells which were at one time called cells of the medullary 
rays, always produce only the same kind of cells; so that the 
secondary medullary rays, when seen in transverse section, 
always reach to the thickening-ring ;' but they never get as 
far asthe pith. The breadth of these rays varies in different 
plants ; in some species they are one cell, in others several 
cells wide; broad and narrow bands sometimes alternate with 
one another ; but they never penetrate through the whole 
length of the plant, as can be easily seen on tangential sec- 
tions (Fig. 474). Both the primary and secondary medullary 
rays are generally easily recognised by the naked eye, and 
form the pattern on the wood. ordinarily known as the 
‘silver grain.’ They consist of parenchymatous cells usually 
elongated in the radial direction (see Figs. 474, 475). 
Both the xylem- and the bast-portion of the vascular 
bundles of the stem of Dicotyledons consist—with the 
exception of the fundamental tissue of the medullary 
rays—of elements which may be arranged into three 
