156 TRACHEM. 



elements one with another, and upon certain phenomena of structure of the walls, 

 which will be stated in the description of the latter. 



The walls are, as already indicated, always interruptedly thickened, the 

 thickening masses following the well-known rules for cell-membranes ^ and being 

 either pitted, or forming fibrous bands, or both. The form of thickening is either 

 uniform over the whole wall of an element, and even of many contiguous elements, 

 or it varies at different points of one wall-surface, or on different sides of one tube, 

 according to the nature of the neighbouring tissue : these differences are found 

 to be especially frequent in the secondary wood (Chap. XIV). Vessels in which 

 these varieties occur have been called mixed (Vasa mixta)"- 



That part of the wall of the Tracheae which is slightly or not at all thickened is 

 always a very delicate, almost immeasurably thin film. 



According to the form of the thickening mass there are distinguished — 



(1) Trachea with fibrous thickening bands, under which head are ranged — 



(a) Trachea with spiral fibrous thickening (spiral vessels), 



(i5) Trochees with annular fibrous thickening (annular vessels).. 



(f) Trachece with reticulate fibrous thickening (reticulate vessels). 



(2) Pitted or dotted Trachece. 



(3) TrachecE with transverse bars (Tr. trabeculatae). 



The Tracheae with spiral fibrous thickening (included in I. a) were termed true 

 tracheae (Trachdes katexochen) by Mirbel and P. de CandoUe (Organogr. I), owing 

 to a false conception of the structure of these and other forms, while the annular and 

 reticulate vessels were called false Tracheae (fausses trach^es) or striped vessels, 

 Vaisseaux rayds, the latter being moreover confused with pitted vessels. 



The above forms, especially those with fibrous thickening, often merge into one 

 another, so as to form ' Vasa mixta.' The pitted vessels show in many cases, as 

 will be more thoroughly detailed later, protuberances of the inner surface in the form 

 of fibres usually having a spiral course, more rarely in the form of transverse bars, 

 which traverse the cavity, and give the character to the form (3). 



Sect. 37, In the walls with fibrous thickening the strengthening bands extend 

 inwards from the unthickened membrane, usually as relatively narrow flattened 

 ribands, appearing in section of elliptical or rounded-rectangular, or almost quadratic 

 form : in depth (i. e. perpendicular to the surface of the wall) they are less, or not 

 more strongly developed than in breadth (comp. Figs. 56*, 57). They are frequently 

 very flat, broad plates, in the latter case often broken by short, small slits or de- 

 pressions of the inner surface, e. g. in the spiral or annular tubes of Commelina 

 tuberosa^; rarely they are deeper than they are broad, e.g. the closely-wound fibres 

 of the later developed spiral vessels in the stem of Artanthe elongata and other 

 woody Piperaceae, and especially the annular and spiral bands, like sharp fluting, 

 which protrude far into the cavity in the Tracheae of the stem of many Cacteae *, and 



' Hofmeister, Pflanzenzelle, § 25. — Sanio, /.c. 



' Compare P. Moldenhawer, Beitr. p. 185 ; Von MoM, Verm. Schr. pp. 278, 279. 

 ' Von Mohl, Ueber den Bau der Ringgefasse, Verm. Schriften, p. 285. 



' Schleiden, Mem. pres. Acad. St. Petersbourg, ser. VI. tom. IV.— Compare Grundziige I. 

 (3 Aufl.) p. 259— Trecul, Ann. Sci. Nat. 4 ser. tom. II. pi. 19. 



