CHAP. II. 
EXOGENOUS STEMS. 
93 
trunks, wliicli proves that it still continues to retain its 
vitality when that of the surrounding parts has ceased. 
The vascular system of a stem one year old consists of a 
zone of wood lying between the pith and the bark, lined in 
the inside by the medullary sheath, and separated into wedge- 
shaped vertical plates by the medullary rays that pass through 
it. All that part of the first zone which is on the outside of 
the medullary sheath is composed of woody tissue and vessels 
intermixed in no apparent order ; but the vessels are generally 
either in greater abundance next the medullary sheath, or 
confined to that side of the zone, and the woody tissue alone 
forms a compact mass on the outside. The second year 
another zone is formed on the outside of the first, with which 
it agrees exactly in structure, except that there is no medul- 
lary sheath ; the third year a third zone is formed on the 
outside of the second, in all respects like it ; and so on, one 
zone being deposited every year as long as the plant con- 
tinues to live. As each new zone is formed over that of the 
previous year, the latter undergoes no alteration of structure 
when once formed : wood is not subject to distension by a 
force beneath it, as the bark is; but, whatever the first ar- 
rangement or direction of its tissue may be, such they remain 
to the end of its life. The formation of the wood is, there- 
fore, the reverse of that of the bark ; the latter increasing by 
addition to the inside of its strata, the former by successive 
deposits upon its outside. It is for this reason that stems of 
this kind are called Exogenous (from two Greek words, sig- 
nifying to grow outwardly). According to Dutrochet, each 
zone of wood is in these plants separated from its neighbour 
by a layer of cellular tissue, forming part of the system of 
the pith and bark; but although this is true in certain plants, 
such as arborescent nettles and others, it is by no means a 
general law. 
After wood has arrived at the age of a few years, or some- 
times even sooner, it acquires a colour different from that 
which it possessed when first deposited, becoming what is 
called heart-icoocl^ or duramen. For instance, in the beech it 
becomes light brown, in the oak deep brown, in Brazil wood 
and Guaiacum green, and in ebony black. In all these it 
