Vegetable Organography and Physiology. 51 
been subjected to the most powerful microscopic observation. 
They possess another wonderful faculty, which is a self- -produc- 
tive quality. Hach vesicle is capable of generating many others 
within itself. 
The woody fibre differs from the cellular tissue, in having its 
vesicles considerably elongated and pointed or wedge-form at their 
extremities. 'The sides of the woody fibres possess a much greater 
tenuity, and are more firm and elastic than the cellular tissues. 
Collected into parallel bundles, and wedged together by means of 
their pointed extremities, they afford strength and support to the 
vegetable fabric, and have, therefore, been denominated the 
“skeleton of the plant.” The ascending sap is transmitted 
through these vessels, yet, like the vesicles of the cellular tissue, 
they have no visible pores. 
» The spiral vessels, like the woody fibre, appear to have origi- 
hated from the simple cell. Like the former, too, they are elon- 
gated tubes; but they possess within their tube, a spiral, woody 
fibre, whose coil seems destined to preserve the integrity of their 
calibre. The office of the spiral vessels is not fully known; but 
as they very generally contain air, and, with the exception of the — 
roots, pervade almost every part of the vegetable system, their | 
function is, undoubtedly, connected with the respiration of the — 7 
plant. “A very curious analogy to this structure,” says Mr. Car- 
- penter, “is exhibited in the trachee, or air tubes of insects, which 
ramify by minute subdivisions through the whole of their bodies. 
These tubes are formed, like the spiral vessels of plants, of an ex- 
ternal membrane, distended by spiral fibre, which is coiled with 
the most beautiful regularity.”* ‘ 
The ducts, which have been spoken of as being a modifica- 
tion of the spiral vessels, differ from the latter, in having no coiled 
fibre within their canal. Their sides are transparent, and are 
studded with minute dots, which give to them the appearance of 
* perforations. Like the preceding tissues, however, they are des- 
titute of all visible pores or openings. By some physiologists it 
is supposed that the dotted ducts serve to convey sap along the 
Stem of the plant; others regard their functions, like the spiral 
Vessels, to be connected with the respiratory system. 
* Principles of General and Comparative Physiology, by Wm. B. Carpenter. 
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