164 



TRACHEJE. 



attached on the other to the thickened lateral wall of the tracheide. In the tracheides 

 of the leaves of Juniperus (Fig. 62), their points of attachment and origin are 

 especially the thick swollen margins of the bordered pits, in the Lycopodia the 

 spiral or reticulate bands with which the lateral wall is thickened. At the corners 

 of the vascular bundles of the leaves of Biota orientalis the swollen margin of the 

 bordered pits is often elongated into blunt cones, which protrude into the cavity, 

 but here end blind, without branching or coalescing with one another or with the 

 opposite wall. As a rare and anomalous phenomenon Sanio ^ found single simple 

 bars, stretched transversely from one wall-surface to the opposite one, in single 

 tracheides of the wood of Hippophae rhamnoides, and Pinus sylvestris : in the latter 

 they are stretched between the tangential walls, and where they occur at all they 

 traverse in the same direction the entire length of long radial series of tracheides, 

 as far as the cambial zone. (Comp. Chap. XIV.) 



Fig. 62.— Juniperus communis ; leaf, FIG. 63. — The same (225) ; vascular bundle, .f xylem, csingle sclerenchymatous fibre 



transverse section (600), f parenchy- at the outer limit of the phloem; / margin consisting of tracheides with bordered pits 



matous cell ; next to it tracheides of the and transverse bars. The parenchymatous cells near and between the latter are 



comer of the vascular bundle, with bor- shaded with dots. 



dered pits, and reticulately-branched 

 transverse bars. The parts lying below 

 the surface of section focussed are 

 shaded. 



Further details on the structure of the walls of the Tracheae will be described in 

 later chapters, especially the Vlllth and XlVth. 



Sect. 40. The wall of the Tracheae, whatever be its structure, is in the one series 

 of cases a completely closed membrane {Tracheides) ; or it is broken through at the 

 limiting surfaces between elements placed serially one above another, and originally 

 completely closed, the series thus coalescing to a continuous tube, which is called a 

 vessel. The tracheides therefore differ from the vessels only in the absence of the 

 holes which occur in certain of the walls, and thus connect the cell-cavities. Tran- 

 sitions between them occur in the secondary wood of Dicotyledonous plants (comp. 

 Chap. XIV), e. g. Leguminosae, inasmuch as with otherwise similar characters the 

 holes are absent in one case and present in another. Holes are also to be found in 



* Botan. Zeitg. 1863, p. 117. — Pringsheim's Jahrb. Bd, IX. p. 59. 



