DUCTS. 



23 



lining. When viewed by transmitted light, it appears as if 

 riddled full of holes. 



a. It is of two kinds ; 1st, 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 arc the largest vessels in the vegetable fabric ; and their open mouths 

 are particularly discernible in the cuttings of the oak, cane, &c. 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 tme spiral vessel much resembles the woody fibre in 

 forai, being a long, slender tube, tapering each way, but is thin- 

 ner and weaker. Its peculiar mark is an elastic, spiral fibre, 

 coiled up witliin it, from end to end. 



b. The spiral tliread is usually single, sometimes double, triple, &c In the 

 Chinese pitcher j^lant, it is quadruple. (Fig. !,/•) 



c. In size, spiral vessels are variable. Generally their diameter is about -j^'^^ 

 of an inch ; often not more than -juVTr- 



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 tlie 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. 



f. Duds are membranous tubes, with conical or rounded ex- 

 tremities, their sides being marked "wath transverse bars, rings, 

 or coils, incapable of being unrolled without breaking. 



FIG. 2. — Fonns of tissue, &c. ; a, annular ducts; 6, spiral and annular at intervals; e, 

 laticiferous tissue ; f, storaata of iris — vertical section, d, rf, green cells at the orifice ;/, /, 

 cells of the parenchyma, e, air-chamber ; g-, g-, view of epidermis and stomaia of >Ttcx;a, h, 

 stomata closed ; small, luminous bodies in the cells. 



