TRACHEA. 



^57 



of the leaves of many Mesembryanthema, e. g. M. stramineum. A less common form 

 of the fibres, corresponding to the bordered pits, is that of which the section has the 

 outline of a short-armed recumbent [— , while the fibre is attached to the thin wall by 

 the free end of the single (here horizontal) arm. This is the case in the closely- 

 wound spiral tubes, which show transitional forms to the reticulate, and, in many 

 woody stems, are first fully formed when the extension of the internode is ended, 

 as is the case in Artanthe elongata, Nerium, Convolvulus Cneorum. The single arm is 



FIG. s6». — Piece of an annular vessel from the stem of Zea Mais, h the thin wall, on which the limits of the adjoining cells are 

 visible, r annular fibres, y one of these cut through, iy the strata of the same (350), From Sachs' Textbook. 



Fig. 57. — Saururus cernuus. Piece of a radial longitudinal section through a vascular bundle of the herbaceous stem ; r in- 

 most, narrow, distorted annular-vessel. To thejeft of this successively (i) spiral vessel, with loosely wound single fibre, which 

 in two places runs back into itself, so as to form a ring ; the thin wall between the spirals of the fibre has given way ; (2) spiral 

 vessel with very narrow flattened curve, cut longitudinally in half, with exception of the upper margin ; (3) scalariform reticulate 

 vessel ; yselerenchyraa (or bast) fibres. The curves of the spiral fibres rise in the drawing in the opposite direction to the course 

 they really pursue (375). 



in these cases almost always smaller than the two others; in the above-named Artanthe 

 it is very inconspicuous compared with the other strongly-protruding parts. 



The Trachea with spiral fibrous thickening show considerable variety in the 

 number of the fibres, and the steepness and direction of their coils. Their 

 number is often only 1-2 in the narrow tubes, which are first formed when the 

 differentiation of tissues begins, in others 4 or more, and it rises in many cases, e. g. 

 the petiole of Musa, to 16-20. The steepness of the coils is greatest in those tubes 

 which are developed earliest, before the extension of the part to which they belong 

 has ceased : since in these the coils are separated from one another by the 



