. L'anatomie comparée des organes végétatifs des phanérogames marines et de fougères ;. Anatomie végétale ; Fougères. 48o Les modifications secondaires, les différentes formes de perforation de la paroi transversale varient selon les cas particuliers (espèces). Les navires sont relativement minces-vi ?composé dans la plupart des cas, même dans les bois très dur et woodsj sont souvent très délicats (Camellia japonica). Plus rarement l'épaisseur de leurs parois est égale à celle de la paroi plus épaisse des éléments qui les accompagnent, e. g. Fraxinus excelsior, Ornus, Nerium oleander, Piperaceae, Convolvulus Cneorum. Les Tracheides (Fig. 205)

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. Comparative anatomy of the vegetative organs of the phanerogams and ferns;. Plant anatomy; Ferns. 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 directio

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