VESSELS. 



iS'J 



arranged : thus the narrow, closely-grouped transverse slits in the ladder-like oblique 

 walls of the pitted vessels in the wood of the Betulacese, Ericaceae, of Corylus, 

 Carpinus, Pteris aquilina ; the round openings of the oblique walls of Ephedra 

 arranged in 1-2-3 rows, &c. Rarely the slits are parallel to the longitudinal axis of 

 the vessel : e. g. vessels of Hieracium vulgatum, Onopordon Acanthium, in which 

 irregular reticulate openings also occur. In an Avicennia Sanio found the hori- 

 zontal septum surrounded by a sharply-marked thickened margin, and the whole 

 remaining surface covered with many irregular, round or slit-shaped, bordered 

 openings ^ There is no constant relation between the form of thickening of the 

 lateral wall and the form of perforation. Nevertheless most vessels with fibrous- 

 thickening have round openings, and very many vessels with bordered pits have 

 ladder-like perforations. In pitted vessels, however, simple openings are also 

 frequent, and Sanio found ladder-like perforations in the spiral vessels of species of 

 Casuarina, Olea europsea, and Vitis. 



In thin-walled vessels — such as most of those with fibrous thickening, and 

 thin-walled pitted vessels, e. g. in the wood of Betulacese and of Tilia — the margin 

 of the opening of the septum is 

 smooth and thin, corresponding to 

 the margin of very flat, not bor- 

 dered pits. In thick-walled vessels 

 it is thicker, and has the structure 

 of a pair of corresponding bordered 

 pits opened by disappearance of the 

 Hmiting lamella, and with but small 

 diff"erence of width between the pit- 

 cavity and the wide orifice of the 

 pit : it consists therefore of two 

 acutely-diverging lamellae. In many 

 vessels this structure as of a bor- 

 dered pit is extremely striking, e. g. 

 in the large solitary openings of the 

 pitted vessels in the wood of Nerium, 

 Fraxinus, Convolvulus Cneorum (Fig. 

 64), Pirus torminahs ^ in the serially- 

 arranged round openings of the ves- 

 sels of Ephedra (Fig. 60, g), and the 

 small slits of the scalariform ves- 

 sels of Pteris aquilina (Fig. 61). It 

 also occurs very plainly in the thick 

 and closely-wound spiral vessels in the stem of Nerium. In other cases, even when 

 the margin of the opening is very thick, it is often only slightly indicated by a small 

 indentation running over the limiting lamella of the margin : e. g. the pitted vessels 

 of Cucurbita, Juglans, Acer monspessulanum (Dippel, I.e.). The history of develop- 

 ment shows, in the cases of the latter category, that the opening arises by the 



Fig. 64. — Convolvulus Cneorum, wood, transverse section (600); £-, ^ 

 small vessels cut transversely through the oblique septum. The rest are 

 tracheides. At / the very delicate closing membrane of the bordered pit 

 is visible. 



' Sanio, Botan. Zeitg. 1863, p. 121. — Von Mohl, I.e. 

 ' Compare Dippel, Botan. Zeitg. i860, p. 329. 



