DUCTS. 23 
lining. When viewed by transmitted light, it appears as if 
riddled full of holes. i 
a. It is of two kinds; Ist, articulated, having its tubes interrupted by joints and 
partitions, as in the oak, vine, and in the monocotyledonous stems; 2d, continuous, 
without joints or partitions; often found in the roots of plants. 
b. These are the largest vessels in the vegetable fabric; and their open mouths 
are particularly discernible in the cuttings of the oak, cane, &e. It is through 
these that the sap arises to the stem, and is conveyed to the leaves. 
33. VASCULAR TISSUE consists essentially of spiral vessels, with 
their modifications. | 
a. The true spiral vessel much resembles the “woody fibre in 
form, being a long, slender tube, tapering each way, but is thin- 
ner and weaker. ~ Its peculiar mark is an elastic, spiral fibre, 
coiled up within it, from end to end. “ae 
b. The spiral thread is usually single, sometimes double, triple, &c. In the 
Chinese pitcher plant, it is quadruple. (Fig. 1, f.) ; 
c. In size, spiral vessels are variable. Generally their diameter is about rot 
of an inch; often not more than go/o0- 
d. 'The situation of spiral vessels is in the medullary sheath, that is, just around 
the pith ; also in every part which originates from it, such as the veins of leaves, 
petals, and other modifications of leaves, and especially in the petioles, from 
which it may be uncoiled, in the manner above described. (28, a.) 
e. In their perfect state they contain air, which they transmit, in some way, 
from one to another. : 
J. Ducts are membranous tubes, with conical or rounded ex- 
tremities, their sides being marked with transverse bars, rings, 
or coils, incapable of being unrolled without breaking. 
fi 
FIG. 2.— Forms of tissue, &c.; a, annular ducts; b, spiral and annular at intervals; c, 
Jaticiferous tissue ; e, stomata of iris — vertical section, d, d, green cells at the orifice ; 4, 4 
ells of the parenchyma, e, air-chamber; g, g, view of epidermis and stomata of yucca, h, 
a closed ; small, luminous bodies in the cells. 
