TRACHE.E. 169 



Palm stems* (compare Chap. VIII), where they attain a diameter of o-28oM»m 

 (Mauritia armata) to o-562""ii (Calamus Draco) ; in the wood of many climbing 

 and twining plants, e. g. Cucurbita, Cobaea, Phytocrene ^ Ampelideae, in which also 

 the width may rise to 0-3 — o-5'i^ni^ &c,3 f j^g vessels of greatest width are always 

 pitted vessels with short members. 



After what has above been said on longitudinal course and branching, it need 

 hardly be noticed further that in one and the same vessel the width (and with it the 

 form of thickening of the walls) may often change in successive parts of its course, 

 i. e. in its successive members ; for instance, Mohl states the diameter of the above 

 vessels of the Palm-stems at the lower ends of the bundles as o-oii™™. 



As regards the material composing the walls of Tracheae and Tracheides, it is certain 

 that they are, when first formed, cellulose membranes, and that they consist, when 

 mature, of more or less lignified cellulose. The lignification is present to a very varying 

 extent according to the special case ; in hard, firm parts more than in soft, sappy parts ; 

 the Tracheae of delicate foliage-leaves, or of sappy stems, &c., often show an almost pure 

 cellulose reaction. A very remarkable phenomenon, to which Burgerstein has recently 

 again drawn attention, is the surprisingly early appearance of lignification in many 

 vessels. It is beyond the scope of this work to enter minutely into the process of 

 lignification : it cannot at present be exactly stated how far it shows peculiarities in the 

 several organs in question. Reference may therefore only be made here to works upon 

 the subject : the summary of the older results in Hofmeister, Pflanzenzelle, Sect. 30, 

 Kabsch, Pringsheim's Jahrb. Ill, and the newest investigation of A. Burgerstein, 

 Sitzungsber. d. Wiener. Acad. Bd. 70, July, 1874. 



Sect. 42. All Tracheae are alike in the peculiarity that when they are fully 

 formed the protoplasmic body disappears entirely, without leaving any vestiges 

 behind, as is the case in dried-up cells. The membrane alone remains of the 

 components of the cell. The space surrounded by it is filled in the mature tube 

 with very dilute watery fluids, which may here be called shortly water, or with air, 

 or with both together. The large majority of Tracheae are entirely or for the most 

 part filled with air at the time of full development. The extremely thin layer of fluid 

 on the inner surface, which is always difficult to observe, is often beyond anatomical 

 demonstration : even in cases of excessive supply of water in bleeding parts air 

 bubbles occur in the fluid contained in them *. It is only in lateral extensions of the 

 vascular bundles of certain plants (Transfusion tissue. Chap. VIII) and in the endings 

 of bundles that they are exclusively filled with water. The same holds for the rudi- 

 mentary Tracheae of many water plants. 



A remarkable exception to this occurs very generally in plants which contain 

 latex, or resinous, or tannin-containing secretions, whether the latter be stored in the 

 sacs treated of in Sections 33 and 34, or in intercellular reservoirs (Chap. VII). A 

 greater or less number of vessels are often filled in these plants for a greater or less 

 distance with latex, or with some such characteristic secretion. No fixed rule is to be 

 found as to the position of these vessels relatively to the other normal air-containing 

 vessels, or to the secretory reservoirs. How the secretion gets into the vessels is not 



' Compare Von Mohl, Ban des Palmenstammes ; Verm. Schr. p. 142. 

 ' Mettenius, Beitr. zur Botanik, p. 50. 



" [Compare Westermaier u. Ambronn, Lebensweise u. Strnctur d. Schling- u. Kletter-pflanzen, 

 Flora, 1880.] * Compare Hofmeister, Flora, 1858, p. 2. 



