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 tlie monocotylcdonous stems ; 2d, continuous, 

 witliout joints or partitions ; often found in tlie 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, &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 true spiral vessel much resembles the woody fibre in 

 form, being a long, slender tube, tapering each way, but is tliin- 

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

 coiled up within it, from end to end. 



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

 Chinese pitcher plant, it is quadruple. (Fig. 1,/.) 



c. In size, spiral vessels are variable. Generally their diameter is about 

 of an inch ; often not more than -g-ijinj"- 



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. 



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

 tremities, their sides being marked with transverse bars, rings, 

 or coils, incapable of being unrolled witliout breaking. 



1000 



FIG. 2. — Forms of tissue, &c. ; a, annular ducts; b, spiral and annular at intervals; c, 

 laticiferous tissue ; e, stomata of iris — vertical section, d, d, green cells at the orifice ; /, _/", 

 cells of the parenchyma, e, air-chamber ; g, g, view of epidermis and stomata of yucca, A, 

 Btomata closed ; small, luminous bodies in the cells. 



3 



