158 



TRACHEA. 



extension which the tube itself undergoes ^ By this process a spontaneous separa- 

 tion (tearing off?) of the fibre from the elongating wall may occur''. If the tube 

 developes later, during or after the close of the extension of the part, the coils are 

 less steep : when several fibres are present they are then arranged at the minimum 

 distance from one another. 



The coils rise in most cases (when seen from without) from right to left, that 

 is like the thread of a left-handed screw, or, according to the terminology adopted in 

 Botany, the spiral is right-handed. The opposite direction occurs in Pinus 

 sylvestris (Mohl) : in the wood of Vitis vinifera, Berberis vulgaris, Artemisia Abro- 

 tanum, Bignonia capreolata, the inmost first-formed tubes are right-handed, the outer 

 later-formed ones left-handed. Where the spiral fibre is interrupted, both the 

 opposite directions of inclination may occur at different heights in one vessel, e. g. the 

 stem of Cucurbita'. 



Not unfrequently, and especially in the closely- wound forms, the spiral fibres 

 are branched, or their coils are connected in a bridge-like manner by oblique or 

 transverse fibrous bands. It is a no less common phenomenon that one fibre at the end 

 of a tube, or at other points, should run back into itself, thus forming a ring. This 

 phenomenon characterises the series of numerous transitional forms between the 

 spiral tubes and the annular and reticulate tubes, while it gives rise in the latter to a 

 number of special forms of the net. It should be added, in connection with the 

 annular tubes, that the distance of the rings from one another follows the same rules 

 as the steepness of the inclination of the spiral fibres. Besides those above mentioned, 

 there is ampng the reticulated tubes a varied series of special conformations of the net. 

 Reticulated tubes, the meshes of which are elongated transversely, and arranged on one 

 surface of the wall in a row one above another, being thus comparable to the rungs of 

 a ladder, are called ladder-like or scalariform vessels, and have frequently been con- 

 founded with the pitted vessels, which show a like surface of wall. (Comp. Fig. 56*.) 



Individual peculiarities of the tracheides in the sheath of the roots of Orchidaceae 

 ■will be brought forward again in Sect. 56. 



Sect. 38. It is well known from the general doctrine of cells, that it is only the 

 relative size of the unequally thickened parts of the membrane which gives rise to. a 

 general distinction between reticularly thickened and pitted membranes, and that 

 therefore there is no sharp limit between these two forms. The wall of the pitted 

 tracheides shows sometimes simple pits, i.e. not bordered, sometimes bordered pits^. 



The term pit is applied to a gap in the internal thickening of the wall, this gap 

 being closed externally by a piece of membrane which is. only slightly or not at all 

 thickened. It is in fact a canal varying in length according to the extent of the 



1 Von MoM, Veget. Zelle, p. 26. 



^ Compare Sachs, Textbook, 2nd Engl. Ed. p. 90. With this must.not be confused that 'unrolling' 

 of spiral fibres, which occurs when a part is torn, and tfee often-cited ' power of unrolling ' of spiral 

 vessels. The latter phenomenon comes about simply \iy the rupture of the delicate unthlckened 

 membrane when the part is torn, while the tough fibre, to which the delicate and easily overlooked 

 tatters of the raptured wall are attached, is drawn out. 



= Von Mohl, Verm. Schriften, pp. 287, 321. — Sanio, I.e. p. 124. 



* [Cf. Rnssow, Zur Kenntniss des Holzes, insonderheit des Coniferenholzes. Orig. communica- 

 tion to Bot. Centralblatt, Nos. 1-5, 1883, in which paper the other principal writings on this subject 

 are referred to.] 



