CHAPTER IV. 



TRACHEA. 



Sect. 36. Under the above name all those tissue-elements may be grouped 

 which have the following characters: (i) their walls become thickened as they 

 differentiate from the meristem, and lignified to a variable extent, while the 

 thickening is arranged in a fibrous manner, or with bordered pits, or rarely with 

 transverse bars ; (2) almost simultaneously the whole protoplasmic body and 

 organised contents of the cells, which are being transformed, disappear altogether, 

 their place being taken by air, or by clear watery fluid. The larger and more 

 elongated tubes falling under this definition were distinguished even by the old 

 anatomists S as Trachea, Vessels, Tubes (Vasa, Tracheae, Fistulas) ; more recent 

 investigations ^ have led to the recognition of two subdivisions, which, according to 

 Sanio, rriay be distinguished as (i) Tracheides, and (2) Vessels (Vasa) or Trachea, in 

 the narrower meaning of the word. I shall use the name Tracheae in this book only 

 as a collective term for both, and specially also in those cases where it is not 

 certainly decided whether a tube belongs to the one or to the other subdivision. 



As will be thoroughly discussed in later chapters, the chief points of occurrence 

 of the Tracheae are the vascular bundles and woody bodies. It may however be at 

 once pointed out here by way of explanation that the above localities are by no means 

 the only ones in which Tracheae are found. Tracheides are found solitary in the 

 parenchyma (Sect. 55) in many plants, and form the root-sheath characteristic of the 

 aerial roots of Epiphytic orchids'. 



Vessels and Tracheides correspond in the general points of structure : both alike 

 have very various special forms ; transitional forms are to be found between them, 

 which will be described more especially in the secondary wood (Chap. XIV). The 

 distinction between them depends entirely upon the mode of connection of the 



' Malpighi, Grew, Anat. Plant. — Compare Treviranus, Physiol. I. p. 82 ; Link, Philosoph. Bot. 

 p. 90, &c. 



^ Sanio, Botan. Zeitg. 1863, p. 113.— Caspary, Monatsbr. d. Berliner Acad. July, 1862.— Caspary 

 terms the organs here called Tracheides, as far as they were the subject of his investigations, 

 ' Leitzellen.' 



' It need hardly be said that in extending the term Trachea to all tissue-elements which 

 coiTespond in the above peculiarities of structure, without regard to their place of occurrence, it 

 includes also the well-known air- or water-containing, usually fibrous- thickened elements of the leaves 

 of Sphagnum and Leucobryacese, they being in fact for the most part Tracheides. Compare on 

 these elements of the mosses, Vgn Mohl, Vei-m. Schr. p. 294, and Schimper, Monograph, d. Torf- 

 moose. 



