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INTRODUCTION TO BOTANY. 55
Medullary canal, Canalis medullaris. The cylindrical
cavity containing the pith.
Medullary sheath, Vagina medullaris. The internal rank
of fibres immediately next to the pith.
Medullary rays. Silver grain. Radii. Productiones. In-
sertiones meduUares. Vertical flakes, similar to the pith,
pushed out towards the circumference; appearing, in a
transverse section of a stem, like the spokes of a wheel.
PI. 1, fig. 2, e. It is in the direction of these rays that
wood, however hard, may be easily riven.
Ligneous portion. Corpus ligneum. Lignea portio. The
part of a stem between the pith and the bark, by which the
juices pass from the root to the leaves. PL 1, fig. 2, b to i.
In monocotyledon plants it composes the whole of the
stem. PI. 1, fig. 4, a to d.
Wood, heart. Lignum, — xylon. That part of the
ligneous portion of a stem which has acquired its utmost
hardness : in dicotyledon plants it forms the centre of the
stem, in monocotyledon plants the circumference.
Alhurnum, Alhurna. That part of the ligneous portion
of a stem which has not acquired its utmost hardness, and
is generally of a paler colour than the rest.
Woody layers. Spurious grain. Strata lignea. Involucra
lignea. Zones formed round the pith or centre of
a stem, appearing in a transverse section like concentric
circles, usually supposed to denote the growth of each successive
summer that the plant has existed. PI . 1 , fig. 2, b, g, h.
Bark, Cortex, — derma. A part of the stem (and root)
of dicotyledon plants, which encloses the woody portion,
separating easily at certain seasons, or by maceration, and
through which the juices of the plant do not pass in going
from the root-sponges to the leaves. PI. 1, fig. 2, a, b.
Cortical layers. Strata corlicalia. The layers, or con-
centric circles to be observed in the bark. PI. 1, fig. 2,
a, b, c, d.
Inner bark. Bast. Liber. The internal cortical layers,
which may be easily separated from the others. PI. 1,
fig. 2, b to d.
Corky substance, Stratum cellulosus, Complexus cellulosuSf
Parenchyma. A layer of cellular tissue, lying on the out-
side of the cortical layers, and in some plants very thick.
Epidermis, Epiderma. A membrane which covers every
part of the plant as well as the stem.