1 68 TRACHE.^. 



disappearance of the limiiing lamella in the face of a young pair of wide corre- 

 sponding bordered pits. 



Since the structure of the bordered edges of the apertures corresponds to that 

 of closed bordered pits, the practical decision whether in a given case an open or a 

 closed pit is present is necessarily extremely difficult, if the parts of the septum 

 in question be small, and resemble closely the bordered pits of the lateral walls 

 in form and size ; and the difficulty is the greater, since in such cases intermediate 

 forms between the open and closed bordered pits — which are never exactly alike — 

 occur at the limits between the septum and the lateral walls. The apertures at 

 the middle of the septum in the pitted vessels of Ephedra (e.g. Fig. 59,/") are 

 larger than the closed bordered pits of the lateral walls. But at the margin of 

 the septum there occur not unfrequently round bordered pits (Fig. 59, x), which, 

 .though resembling the apertures in form and size, are closed like the pits of the 

 'lateral wall. In the large scalariform vessels in the rhizome of Pteris aquilina 

 (comp. Fig. 61) the transverse, bordered, slit-like pits of the lateral walls are always 

 closed. The strongly-inclined septa or terminal fiices of the obliquely pointed 

 members of the vessels show a quite similar ladder-like series of slit-shaped pits, 

 like those of the lateral walls, but with the difference that here the slit-like pits 

 are wider, and the thickened intervening bands which separate them are thinner 

 than on the lateral walls. In the middle of the septum the slits are open, as is 

 plainly shown in sections through vascular bundles which have been injected with 

 glue and then dried, if the sections be then treated with water and the glue dis- 

 solved. The bands between the slits then separate from one another (Fig. 61, D,/). 

 Towards the ends of the intervening wall the pits are, on the other hand, of the same 

 width as the open ones, but closed by a limiting lamella : on the corners of the 

 lateral walls they become gradually narrower, and like those of the lateral walls. 



The vessels are not unfrequently branched, two or more series of members 

 being attached laterally to one member. When such branches run parallel or 

 converge, they may attach themselves again laterally to a single series of members ; 

 a vessel may thus be in its longitudinal course alternately single and double ^. 



As regards the absolute size of the vessels, there is nothing to oppose the view 

 that their length may equal that of the whole plant, or at least may be very great. At 

 all events, on following the vascular bundles through long distances, member 

 is found attached to member, while blind-ends are rare, except in the ends of the 

 peripheral expansions of the plant. The width of the vessels is extremely unequal, 

 and changes variously according to the point of their occurrence in a given plant, 

 and according to the single species or genus. It may be said generally that the 

 diameter does not on the average exceed that of narrow fibrous cells in those vessels 

 which appear first in stems and roots, before the extension is complete (spiral and 

 annular), and in those which traverse the nervation of the leaf. Those formed in 

 stems and roots at the end of the process of extension, or subsequently, may in many 

 cases attain much greater width, while this does not prevent others of the smallest 

 calibre occurring with or near the former. For examples of this see Chap. XIV. 

 Vessels of greatest width occur in the central part of the vascular bundles of many 



* Von Mohl, Palm. Stnictuta; Verm. Schriften, p. 142. 



