48o 



SECONDARY CHANGES, 



The different forms of perforation of the transverse wall vary according to the 

 particular cases (species). 



The vessels are relatively thin-vi?alled in most true woods, even in very hard woodsj 

 and are often remarkably delicate (Camellia japonica). More rarely the thickness of 

 their walls is equal to that of the thickest-walled of the elements which accompany them, 

 e. g. Fraxinus excelsior, Ornus, Nerium Oleander, Piperaceae, Convolvulus Cneorum. 



The Tracheides (Fig. 205) either constitute the only tracheal elements of the 

 wood (Coniferae, Wintereae), or they occur together 

 with other tracheal organs, especially vessels. They 

 are characterised as tracheides by the properties 

 indicated in Chap. IV. The structure of their wall is 

 in the case of the Coniferee and Wintereae that of pitted 

 vessels with bordered pits ; in most Taxineae there are 

 also spirals on the inner side of the wall. In the 

 second case the same holds good, with the addition 

 that they then resemble the members of the narrower 

 vessels of the same wood, either in all points, except 

 the vascular perforation, or at least in possessing 

 similar bordered pits to those of the vessels belonging 

 to the same wood. As regards the spirally or an- 

 nularly thickened inner layer, they usually agree with 

 the vessels accompanying them, but not always: in 

 Pyrus communis, Sorbus Aucuparia, and Staphylea 

 pinnata the vessels have spirals, the tracheides not, in 

 Philadelphus coronarius the converse is the case. In 

 particular tracheides of many plants, we find, as an 

 exception, isolated thickenings of the wall, projecting 

 inwards in the form of blunt cylindrical pegs, or of 

 bars running transversely from one side to the other. 

 Both forms were observed by Sanio in Hippophae 

 rhamnoides, the latter in Pinus silvestris', and casually 

 by me also in Drimys Winteri. The transverse bars 

 lie in the radial direction, at least in Pinus and 

 Wintera, and are continued as a single bar through 

 many elements of a radial row. Peculiar transverse 

 lines, which Sanio found on macerated tracheides of 

 Casuarina (I.e. p. 117), still require explanation. 



Where tracheides occur together with vessels or 

 sclerenchymatous fibres, or both, two extreme cases may be distinguished as 

 regards their form, namely, on the one hand, those which on the whole resemble 

 the members of the smaller vessels in length and width, and abut on one another 

 with a relatively slight inclination of their terminal surfaces; on the other hand, 

 more elongated, ' fibre-like ' forms, with long acuminate ends, sometimes even 

 forked (Hippophae, Casuarina torulosa, Staphylea pinnata), which penetrate between 

 one another and between the non-equivalent elements. 



FIG. 205.— Cytisus Laburnum ; tangential 

 section tlirough the same autumn wood as 

 Fig. 198, p. 465. s intermediate cells, tn — 771 

 medullary ray ; in the second cell from the 

 top is a peg-shaped thickening of the wall. 

 Tlie medullary ray is bounded on the left by 

 an unperforated tracheide, on the right by a 

 narrow vessel ; at ^ is the perfcration of the 

 transverse wall of the latter. The lower 

 longitudinal wall of the traclieide to the left 

 was preserved in the preparation ; the upper 

 one, all but a very small piece, was removed 

 in cutting the section ; in the other tracheides 

 and vessels the longitudinal wall facing up- 

 wards is drawn, the spiral fibres being shown 

 in reversed direction, and the really. bordered 

 pits with only a single outline. 



Compare above, p. 164. 



