BORDERED PITS. l6l 



margin ; in Ephedra, which is the most striking instance I know of, it is shaped like 

 a flat biconvex lens ; in other relatively small, or at least narrow pits (Cassyta pani- 

 culata, vessels of Nerium, Pteris aquilina, &c.), it appears as a hardly perceptible 

 swelling. The thicker portion always lies, like a lid, upon the outer aperture of 

 one of the pit-canals. 



The corresponding bordered pits of neighbouring Tracheae are accordingly 

 closed by the limiting lamella, which occasionally remains plane, but as a rule is 

 applied to one wall of the pit-cavity. On account of its delicacy, and the small size 

 of the whole pit, the limiting lamella, in the form in which it usually occurs, has not 

 hitherto been clearly recognised. In contradiction to Hartig ^ alone, and following 

 the statements of Schacht and Dippel ^, the pit-cavity was regarded as being in the 

 mature state in open communication on both sides with the adjoining cavities of the 

 tubes, while the few cases in which the limiting lamella was observed were considered 

 as exceptions. Sanio ' has recently clearly proved that in Plnus sylvestris the case is 

 as stated above. I find his statements confirmed in all cases subjected to exact 

 investigation, both in the tracheides of the wood of that tree, and in those of Abies 

 pectinata, excelsa, Juniperus communis ; also in the tracheides, and lateral walls of 

 the Tracheae of Ephedra, and Welwitschia : further in the lateral walls of the 'scalari- 

 form vessels' of Ferns (Pteris aquilina) ; the tracheides of the secondary wood of 

 Dracaena, Cordyline paniculata; the Tracheae of the wood of Convolvulus Cneorum, 

 Statice monopetala, the large-pitted vessels of the wood of Cass)fta (C. paniculata, 

 R. Br.), Nerium Oleander, &c. Extremely good preparations, which are not always 

 easy to obtain, always show the case as described : it should then be characterised 

 at least as the regular condition, and that which is distributed over the most various 

 divisions of the vegetable kingdom. Further investigations must show whether ex- 

 ceptions occur. 



While the above fundamental conditions of structure remain constant, the 

 special form of the bordered pit is very variable (compare the Figures 58-62, and 

 what follows in Chaps. VIII and XIV) ; firstly, according to the length of the canal, 

 which depends upon the extent of thickening of the walls ; this has already been 

 mentioned above ; secondly, according to the special form of the pit, and of the 

 canal with its outer and inner aperture, as seen most distinctly in surface view of 

 the wall, and according to the relative size of the diameter of these parts in each pit. 



All these parts have the forms generally characteristic of pits, which (in surface 

 view of the membrane) vary in individual cases between a circle and a narrow slit. 

 In the same pit all the parts are alike in form, or very similar, as is the case in the 

 circular pits of the tracheides of the wood of Pinus, and the slit-like ones of most 

 scalariform vessels, thus giving rise to the appearance of the bordered pit in surface 

 view as two or three concentric outlines, which differ only in size (e. g. Fig. 58, 61 £). 

 On the other hand, the form of the parts may differ in the same pit, either so that they 

 all differ from one another, or one from the rest, and this may occur in all possible . 

 combinations. Thus there is a narrow elliptical inner aperture, and a circular outer 

 aperture of the canal, which diminishes greatly towards the outside, with irregularly 



' Compare especially Botan. Zeitg. 1863, p. 293. 



' Schacht, Be maculis (pits), &c. Programm. Bonn, i860.— Dippel, Botan. Zeitg. i860, p. 329. 

 ' Piingsheim's Jahrb. Bd. IX. [See also Sachs, in Arbeiten des Bot. Inst, in WUrzburg, II. p. 294.] 



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