PHYSIOLOGY AND STRUCTURE OF TREES AND SHRUBS. 117 
exclusively to the first division, Exogens. As already stated, the 
stems of exogenous plants, when in their normal state, are 
known by having pith, medullary sheath, medullary rays, woody 
' tissue and bark, as is shown on Plate 5, which represents a 
section of the stem of the Black Italian Poplar. The pith 
occupies the centre, and is seen as small circular discs. On 
the outer edge of the pith is the medullary sheath, composed 
of true spiral vessels presenting a dark irregular line or band, 
having a zone of wood on the outside of it through which the 
medullary rays are seen projecting into the sheath, and connected 
with the cambium layer. This latter, which consists of cellular 
tissue, shows itself as a thin dark line lying between the wood 
and the bark. The outer band shown on the slide is the cortex. 
The portion between the latter and the cambium band consists of 
two layers of fibro-vascular tissue, known as the “live” bark. 
As above stated, the pith occupies the centre of the woody 
stem, as is shown by a transverse section of the Vine on Plate 
6, and consists of cellular tissue, having spiral vessels originating 
in the medullary sheath, which itself consists of spiral vessels, 
and surrounds the pith connecting the medullary rays. The pith 
is in direct communication with the leaves through these spiral 
vessels. By the pith, or medullary system, the buds and leaves 
are supplied with oxygen, liberated by the decomposition of 
water, and its office is to nourish the young growth. This done, 
it either dies by “strangulation” or “surfeit.” The section of 
the pith of the Elder, shown on Plate 7, is a very good example 
of pith-cells. Plate 8, showing the first year’s growth of a Vine, 
is a repetition of Plate 6, but showing a much larger medullary 
sheath with its projections into the pith. Plate 9, showing the 
first year’s growth of the pith, wood, and bark of Ficus elastica, 
exhibits the young wood lying upon the medullary sheath. This 
wood is formed by successive layers of organised matter, descend- 
ing from the buds and leaves, interposed and regulated by the 
medullary rays, which, through the spiral vessels, connect the 
pith with the cambium. A section of Quercus Ilex on Plate 10 
shows the woody layers perfected. They consist of vasiform tissue, 
which varies in appearance with the kind of plant producing it. 
The layers are concentric, and seldom or never alter their shape 
after once being fully formed. Their density is altered, however, 
by the secretions. These secretions form the duramen when 
deposited in the wood. When deposited in the bark and the 
