]66 TRACHE.E. 



separable from one another, iheir limits being marked by their margins, which are 

 always permanent, often also by other portions of the perforated dividing wall : these 

 portions have the structure of a thickened double cell-membrane, and consist of two 

 thickening plates and a simple limiting lamella between them. Schultze's mixture, 

 or hot solution of potash, destroys the limiting lamella, and thus separates the mem- 

 bers from one another. 



The form of a member of a vessel is as a rule cylindrical or prismatic, the 

 breadth being throughout almost uniform, or diminishing quite gradually towards one 

 end : more rarely each member widens in the middle to a barrel-shape. The length 

 of a member is usually greater than the diameter : it is very much greater in the 

 vessels with a loose spiral, or in annular vessels, which develop before the extension 

 of a part is complete, and thus grow greatly in length as the part elongates. The 

 members of such vessels as arise after the extension of a portion of a stem or root 

 is complete are composed of short members, these being barely longer or even 

 shorter than they are broad, e. g. the wide-pitted and reticulate vessels of old stems 

 of Cucurbita, Cobcea, Vitis, &c. (comp. Chap. XIV). Successive members of a 

 vessel are as a rule of almost similar form through long tracts, though they often 

 decrease gradually in width. The general form of the vessel may be concluded 

 from these data : such as are composed of short barrel-shaped members were dis- 

 tinguished by the old authors as rosary-shaped : Vasa moniliformia. 



The walls, by which the members of the vessel are in contact with one another, 

 are either horizontal, in which case those of the successive members fit exactly on 

 one another, and together form the septum (comp. e. g. Fig. 3,^), or they are more 

 or less oblique, and the inclined faces of successive members here also fit exactly 

 throughout so as to form the oblique septum (Figs. 59-61) : or the ends are oblique 

 and pointed, and only a part of the opposed faces of successive members is united 

 so as to form a septum, near and above which the pointed end forms a blind and 

 often irregularly-formed continuation. 



The perforation of the septum is always brought about thus : on the delicate 

 primary membrane one or several flat large pits are formed by the typical pro- 

 cess of thickening ; the unthickened parts of the membrane are then at once 

 dissolved and disappear, while the thickened bands of membrane, connected directly 

 with the thickenings of the lateral walls, remain persisient. In almost all cases on 

 horizontal septa, and not uncommonly on oblique ones, there appears one single 

 pit, or one single round or elliptical opening, which then always occupies the 

 greater part of the surface of the septum, and often, especially in thin-walled 

 vessels, the whole surface with exception of a very narrow peripheral band. On 

 the other hand, strongly-inclined septa, and very rarely horizontal ones (Avicen- 

 ijia), retain in most cases several or many openings included within the thickened 

 margin, and separated from one another by thickened bands. These are in some 

 few cases round, e.g. in the Tracheae of Ephedra^ (Fig- 59)i usually they have the 

 form of slits of varying breadth, and arranged parallel in rows, whence the expres- 

 sion ladder-like per/oraled septa (Fig. 61). The slits are usually almost at right 

 angles to the longitudinal axis of the vessel, and the series of them are similarly 



* Von Mohl, Ueber den Bau cl. grossen getiipfelten Gefasse von Ephedra ; Verm. Schr. p. 268. 



