TRACHEIDES. VESSELS. 165 



the elements of the root-sheath of many Orchids (Sect. 56), but these had better be 

 termed generally tracheides, since the connection in rows which is characteristic of 

 vessels is absent. 



The Tracheides are in some few definite cases — ends of vascular bundles, trans- 

 fusion tissue, the root-sheath of Orchids — short, even iso-diametric sacs : as a rule 

 they are of the form of elongated, spindle-shaped, fibrous cells, pointed at the ends, 

 and with round or polygonal transverse section. They usually remain microscopically 

 small, their length, which is a large multiple of their breadth, reaches 016'n'n to 

 about i-oo™": this is the case in the wood of most Dicotyledons^: or it rises to 

 4™™, as in the later annual rings of Pinus '^ : in many cases however they attain 

 great dimensions : the large spindle-shaped spiral and annular tubes in the stem 

 and petiole of Musa and Canna" attain a width of o-o8 to o-io°i™, and are always 

 more than i<"" in length; the spiral tubes of Nelumbium speciosum have, according 

 to Caspary, a length of over 12"=™, and width of 0-567™™. The great majority of 

 Tracheae belong to the category of tracheides : for instance, the tracheal elements of 

 all peripheral ends and expansions of vascular bundles, of the secondary wood 

 of the Coniferse, Cycadese, most elements of the secondary wood of woody Dicoty- 

 ledons, almost all Tracheae of the Ferns, in the widest sense — vessels are only known 

 to occur in Pteris aquilina, and in the root of Athyrium filix femina * — the Tracheae 

 of the vascular bundles in stem and leaf of the Cycadeae and Coniferae ", of many, 

 though far from all Monocotyledons, and numerous Dicotyledons ". Many even of 

 the most striking elements with fibrous thickening, usually described as vessels, 

 belong to this series. To the already-cited examples of Canna, Musa, and Nelum- 

 bium we may, according to Caspary's work quoted above, and referring to this for 

 further details, add the following examples : the ' vessels' in the vascular bundles of 

 Stratiotes aloides (stem), Caladium nymphaeifolium, Pistia Stratiotes, Acropera Loddi- 

 gesii, Aerides odorata, Alisma Plantago, Sagittaria sagittaefolia, Hydrocleis Humboldtii, 

 Musa spec, (vessels in this case in the root), Brasenia peltata, Nuphar luteum, pumilum, 

 Nymphaea alba, gigantea, Victoria regia, Monotropa Hypopitys. A general view of the 

 occurrence of tracheides and of true Tracheae will only be possible when the necessary 

 arduous investigations have been extended over a larger number of cases than has 

 hitherto been the case. 



Sect. 41. A vessel arises from a series of originally-separate cells, placed one 

 above another, by the perforation, at the close of the process of thickening, of the 

 division walls between the members of the series, the latter being then termed the 

 members of the vessel. 



The rows of cells, above indicated (pp. 9 and 11, in Figs. 2 and 4) by the 

 letter v, which extend to the apex of the plerome, and similar ones marked g in 

 Fig. 3, p. 10, are rudiments of vessels. 



The members are also always easily distinguishable in a mature vessel, and are 



' Sanio, Botan. Zeitg. 1863, p. 114. '' Sanio, in Pringsheim's Jahrb. VIII. p. 401, &c. 



' Compare Unger, Anat. und Physiol, p. 171, and p. 218, Fig. 92 b. 



* Russow, Vergl. Untersuchungen, p. 103. 



' Mettenius, Beitr. zur Anat. d. Cycadeen, p. 258. [See also V. Hbhnel, Ueber das Vorkommen von 

 Gefassartig-zusammenhangenden Tracheiden-strangen in Coniferen-holzern, Hot. Ztg. 1879, p. 329.] 



• Caspary, Monatsbr. d. Berl. Acad., Juli, i86a. 



