Vegetable Organogra'phy and Physiology. 51 



been subjected to the most powerful microscopic observation. 

 They possess another wonderful faculty, which is a self-produc- 

 tive quality. Each 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- 

 nated 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 

 plant. "A very curious analogy to this structure," says Mr. Car- 

 penter, " is exhibited in the tracheae, 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 Wra. B. Carpenter. 



