MEDULLARY SHEATH.—WOOD OR XYLEM. 81 
vessels, and wood-cells are developed. As, however, these 
elements of the fibro-vascular bundles increase in number, 
they encroach upon the parenchyma, and thus circumscribing 
the central portion till it assumes the appearance of a central 
continuous column or pith (jig. 182, m), filling the interior of the 
stem, and giving off the medullary rays, 7, as flattened plate-like 
processes which connect the pith with the cellular layers of the 
bark, b. That portion of the parenchyma which thus remains, 
including the pith, medullary rays, and cellular layers of the 
bark, is called the fundamental or ground tissue. 
Instead of continuing to form an uninterrupted column, the 
pith, in after years, owing to the external parts growing rapidly, 
becomes more or less broken up ; and even in many herbaceous 
plants, such as the Hemlock and others, which grow with great 
rapidity, it is almost entirely destroyed, at an early period of 
the plant’s life, merely remaining in the form of ragged portions 
attached to the interior of the stem ; and thus large central air- 
cavities or lacunz are formed. In some plants, such as the 
Walnut (fig. 184) and Jessamine, the pith is broken up regularly 
into horizontal cavities separated only by thin discs of its sub- 
stance. It is then termed discoid. 
The diameter of the pith varies much in different plants. 
Thus it is generally very small in hard-wooded plants, as in the 
Ebony and Guaiacum ; while in soft-wooded plants, as the Elder 
and Ricepaper Plant (Tetrapanaz (Aralia) papyrifera), it is large. 
The diameter not only varies in different plants, but also in 
different branches of the same plant ; but when once the ring of 
wood of the first year is fully perfected, the pith which it sur- 
rounds can no longer increase, and it accordingly remains of the 
same diameter throughout the life of the plant. 
The pith, as we have just seen, is essentially composed of 
parenchyma. It also frequently contains laticiferous vessels, as 
may be readily observed by breaking asunder a young branch of 
the Fig-tree, when a quantity of milky juice at once oozes out 
from their laceration. 
2. The Medullary Sheath (jig. 185, B, d) consists of spiral 
vessels which are situated on the innermost part of the wedge 
of wood which forms the first year’s growth. These vessels do 
not form a continuous sheath to the pith, but spaces are left be- 
tween them, through which the medullary rays pass outwards 
(fig. 182, t). As the spiral vessels are never repeated after the 
first year’s growth, the medullary sheath is consequently the 
only part of the stem in which they normally occur. 
3. The Wood or Xylem.—This is situated between the pith 
on its inside, and the bark on its outer surface (jig. 176, 7), 
and it is separated into wedge-shaped bundles by the passage 
through it of the medullary rays, b. We have seen that in the 
first year’s growth of an exogenous stem the wood is deposited 
G 
