i6o 



TRACHEJE. 



tlie lumen (comp. Figs. 59-60). The cavity of the pit is in most cases originally and 

 often permanently of the form of a plano-convex lens {■ half-lens-shaped '), since the 

 outer surface of the thickening of the membrane, which borders it on one side, is 

 concave, while the unthickened portion of the membrane, which limits it on the 

 other, is flat. The canal is, according to the extent of the thickening of the 

 membrane, either extremely short, so that a sharp-edged opening leads from the 

 lumen of the tube into the cavity of the pit, e. g. in the thin-walled tracheides of 

 the spring wood of Pinus ; or, when the thickening is greater, it is elongated, and 

 widens outwards suddenly into the cavity of the pit, e. g. autumn wood of Pinus, 

 pitted vessels of Nerium, Fraxinus, wood-elements of Convolvulus Cneorum, Pteris 

 aquilina (Figs. 61, 64), &c. 



The above general description of the structure of the bordered pit is said to 

 hold for those uncommon bordered pits, not belonging to our present subject, which 

 occur in certain cells \ and for those on the limiting surfaces between Tracheae and 

 other elements; and.it is clear that between these and the non-bordered pits only 

 the above-described difference of form exists, which corresponds exactly to that 

 between the flattened and the \— shaped fibrous thickenings. , On the surfaces 

 abutting on elements of another order, the bordered pits of the Tracheae either 

 correspond to non-bordered pits on the walls of these, or they are opposite to an 

 unpitted wall. But where Tracheae with bordered pits are contiguous with one 



another, the bordered pits correspond to 

 one another in such a way, that on each 

 limiting surface all the cavities of the pits 

 of the one fit exactly over those of the 

 other. The plano-convex cavities are thus 

 applied to one another in pairs, so as to 

 form the ' lens-shaped pit-cavities ' (comp. 

 Figs. 58-62), and each of these is divided 

 by a thin flat lamella of membrane 

 (the limiting lamella) into two halves. 

 This is the case in the first instance in all 

 investigated cases. Also in rnature Tracheae 

 this condition always remains permanent, 

 as can easily be proved in old wood of 

 Pinus, Ephedra (Fig. 606)^. As a rule 

 however the originally plane limiting lamella 

 grows in surface in such a way, that it be- 

 comes larger than the median plane of the 

 lens-shaped double cavity, and therefore 

 bulges in a convex manner to one side, and comes into close contact on this side 

 with one of the concave walls of the cavity of the pit (Fig. 60 c) ; meanwhile it 

 remains a very delicate film, but is always, in the cases investigated, thicker in the 

 middle than at its margin. In Pinus sylvestris (and its allies), as first shown by 

 Sanio, the thicker part has the form of a relatively broad plaite with a sharply marked 



FIG. 60. — Transverse section through the secondary wood 

 of Ephedra helvetica (600) ; m medullary ray, £'—£ vessel, 

 the section having passed through the oblique wall separat- 

 ing two members, and in fact through the middle of an open 

 bordered pit (pore), and to the left of this, through the 

 margin of a pit Besides this vessels and tracheides are 

 transversely cut at a and 5, through the middle of bordered 

 pits of the lateral walls, which have the limiting lamella 

 thickened in a Icnob-like manner on both sides J at c the 

 thickening is on one side; 



' Compare the figures of the endosperm of Phytelephas (?). Schleiden, Grundziige, 3 Aiifl. p. 232. 

 ' Compare Hofmeister, Pflanzenzelle, p. 175. 



