THE STEM. 39 
increase in thickness at all. When they do so, as in some tropi- 
cal trees akin to lilies, the bundles remain closed, no cambium 
being formed in them. A ring of cambium is formed in the 
ground-tissue outside the original bundles, and in the ring new 
bundles are developed, so that growth here also is really by exter- 
nal additions. It is therefore best to drop altogether the terms 
exogen and endogen, but the adjectives exogenous and endogenous 
may be conveniently retained, as denoting the origin of certain 
organs or tissues. A root, for example, is endogenous, being 
developed from internal tissue layers, while a leaf, as we shall see 
farther on, is exogenous (fig. 5). Before leaving the secondary 
wood, it will be useful to examine, in the light of what has been 
said, the appearances visible in a wooden plank, composed say of 
fir wood, z.e., deal. The ends will be more or less accurate cross- 
sections of the trunk, and will therefore present small segments 
of the annual rings. The pith may possibly be seen as a circle, 
but will be insignificant in size. The medullary rays appear as 
narrow radial streaks. A plank running right through the centre 
of the annual rings will have a number of more or less parallel 
lines running along its faces in the long direction. The pith, if 
seen, will be a narrow longitudinal streak, and, of course, the 
stripes immediately bounding this belong to the first annual ring, 
the next stripes to the second ring, and so on. The medullary 
rays appear as streaks rather broader than before, parallel to the 
ends, and for the most part very discontinuous, since the section 
is not likely to run very far along any one of them. The edges 
of the plank display the ends of the rays, and, if rather oblique, 
may cut through several annual rings, the boundaries between 
which will be irregularly wavy. Most planks are, however, more 
or less tangential, and either side of such a plank will, near its 
centre, cut along a single ring for some distance, till in fact the 
ring curves quite out of the plane of section. There will thus be 
a central strip, running longways, and bounded by straight lines. 
Outside this will be pairs of strips, one of each pair on either 
side, getting narrower and narrower, till near the edge a prac- 
tically radial section is seen. As we pass from the centre, the 
medullary rays will be first cut right across, and then obliquely, 
and lastly in the direction of their length. No mention has yet 
been made of “ knots.” These are simply dead branches of diffe- 
rent age, which in tangential section are cut transversely, when 
their age can be determined by counting the rings they possess. 
Their nature is well seen in radial sections, where they will be 
observed to run out as transverse brown stripes from annual 
rings of different age. We can therefore tell when they began 
to grow. ‘The foregoing description will be perfectly useless with- 
