^82 " PRIMARY ARRANGEMENT OF TISSUES. 



Throughout the entire central portion are distributed rows of tracheides, which 

 in all directions are alternately in connection with one another, and interrupted by 

 rows of parenchyma, or by isolated sclerenchymatous fibres; they are connected with 

 the edge of the vascular bundle in the manner described above. 



In the Abietineae last-mentioned the breadth and thickness of the border of 

 tracheides cannot be more exactly defined than by the statement, that the colourless 

 central portion of the leaf is 2-5 layers of cells thick all round the pair of vascular 

 bundles. In the other cases, where it is more sharply limited, its breadth may be 

 stated as on the average equal to that of the vascular bundle, the number of rows of 

 tracheides in the direction of its breadth being about 5-8. In the direction of 

 thickness, vertical to the latter, the number of rows in the thickest external part is 

 usually on the average 4-5, more rarely, when the border is broader, only 1—2 (Cun- 

 ninghamia, Cedrus, Juniperus communis). In Gingko and Prumnopitys the border 

 is quite feeble, showing only 1-3 tracheides in cross-section. On the other hand, in 

 Podocarpus Meyeriana, so often referred to, it is 1-2 layers thick, but on the average 

 15 rows broad, while in the above-mentioned transverse wing of the sheathing base 

 of the leaf of Thuja gigantea it is 25-30 rows broad. At the end of the bundle 

 the short-celled external edge of the border passes over directly into the short 

 tracheides which form the ending of the bundle itself 



The structure of the tracheides themselves is in general that given in Chap. IV, 

 for this form of tissue. In the living leaf I always found them to contain water, not 

 air. Their lignified walls agree generally in structure with those of the spindle- 

 shaped tracheides which form the edge and the external region of the adjoining 

 xylem, and in structure are identical with or similar to those of the secondary wood 

 of the same species. Iii most cases they have the large round bordered pits universally 

 distributed among the Coniferse (cf. Fig. 183). In many forms, on the other hand, they 

 are smooth and thin, as in Abietineae, species of Cunninghamia and Thuja, often also 

 in Araucaria imbricata and brasiliensis, and in Sciadopitys. Many tracheides in the 

 three species last-mentioned, and all of them in Taxus, Dammara, Gingko, and 

 species of Podocarpus, have reticulated and spiral thickening in addition to more or 

 less numerous often isolated bordered pits, in varying distribution, which for most 

 species requires to be more accurately ascertained. In the leaves of the species of 

 Juniperus investigated (J. communis, Oxycedrus, oblonga, macrocarpa, Mohl, I.e., 

 also J. sabina) the tracheides of the border are distinguished by the transverse 

 bars described at p. 163, in Biota orientalis by the projecting pegs mentioned at 

 p. 164 (Fig. 184). 



The reticulated elements at the circumference of the vascular bundles of the 

 leaf of Welwitschia, represented at p. 335, Fig. 157, must be reckoned, among the 

 borders of tracheides in question, so far as the dead material admits of a decision. 

 They are usually quadrilaterally prismatic, short, almost cubical, while some are 

 elongated and even have tapered ends. Their structure is that of tracheides with 

 a narrow-meshed, reticulated membrane, and with isolated bordered pits between 

 the threads. They are arranged in longitudinal rows, which follow the vascular 

 bundle, and are often irregularly interrupted by parenchyma, but at many points 

 stand laterally in direct connection both with each other, and also with the xylem 

 of the bundle; the latter is the case both at the sides of the longitudinal bundles, 



