BORDERED PITS, ETC. 163 



Saliio (/. c. 125) in the wood of Tectona grandis, Fraxinus, Tamarix, &c. This must 

 originate in the thickening of the membrane lasting longer at the inner side than at 

 the outer, and altering its original direction at a later period. On the vessels of the 

 wood of Mahonia aquifolium Sanio found round bordered pits, arranged in left- 

 handed oblique series, with the inner apertures serially coalescent into long slits, 

 while between these the thickening of the walls protruded inwards in form of 

 spiral bands. 



The arrangement of the bordered pits on a wall-surface differs in no way from 

 the known rules for the arrangement of pits generally. They are arranged on a 

 surface in perpendicular, horizontal, or, especially when of slit-like form, in oblique 

 spiral series : in the latter case the spirals are almost always left-handed : the number 

 of these series varies according to the special cases, and on equal areas it varies on 

 the whole inversely with the size of the pits. We may cite as examples of extreme 

 cases, on the one hand, the usually loose series of large round-bordered pits on 

 each radial face of the tracheides in the wood of Pinus, and the several loose series 

 of large pits on the wide vessels of the wood of Cassyta (Mohl, /. c. Fig. i) ; on the 

 other hand the walls covered with close and small pits, which are found surrounding 

 the large vessels in the vascular bundles of the stem of the Cucumber, the tubers 

 of Dahlia ', many Dicotyledonous woods, as Queircus, Nerium, &c., &c. ; in these 

 cases the margins of the pit-cavities are separated from one another by quite narrow 

 bands or ridges of the wall. 



A special case of frequent occurrence may here be mentioned, viz. the trans- 

 verse, slit-like bordered pits, which are characteristic of almost all Ferns (Fig. 61), 

 and which also appear in many Dicotyledonous woods, as Cheilanthus arboreus, 

 Vitis '', &c., these being arranged like the rungs of a ladder, in one or few longitudinal 

 perpendicular rows on a single wall-surface. The wall-surfaces on which they occur 

 may be called scalariform or ladder-like surfaces, whil^^pese Tracheae, together with 

 the similar reticulate tubes above mentioned', have been termed scalariform or 

 ladder-like vessels, Vasa scalariformia, also scalariform tubes. It is useful to dis- 

 tinguish them from the reticulate vessels with non-bordered, transverse pits, either 

 as bordered scalariform surfaces or Tracheae, or to reserve specially for them the 

 name of ladder-like or scalariform surfaces. 



Sect. 39. As one of the above-described forms of wall-thickening there are 

 found, in some few cases to be cited below, ingrowths of the thickened portions of 

 the membrane, which protrude in a conical or bar-like form into the cell-cavity, or are 

 stretched transversely across it : and those Tracheae in which these are largely deve- 

 loped may be distinguished by the name, introduced above as No. 3 (p. 156), viz. 

 Trachea with transverse bars. The bars are very largely developed in the narrow 

 primary tracheides occupying the corners of the vascular bundles of the stems of the 

 stronger species of Lycopodium, and in the margin of the vascular bundles of the 

 leaves of Juniperus* (comp. Chap. VIII). They are here somewhat flattened 

 cylindrical fibres, branching irregularly on all sides, and with the branches connected 

 on the one hand one with another to form a net spread through the cavity, and 



' Compare Sachs, Textbook, 2nd English Ed., p. 26. '' Von Mohl, /. c. 



' Compare Link, Elem. Phil. Botan. Ed. I, p. 95 ; Von Mohl, Veget. Zelle, p. 27 ; Unger, 

 Anatomie und Physiologic, p. 172. * Compare Von Mohl, Botan. Zeitg. 1871, p. 12. 



M 2 



