CHAPTER VII r. 

 TRACHEiE AND SIEVE-TUBES. 



I. Trachea; and sieve-tubes outside the vascular bundles. 



Sect. 55. Both the above organs are, as above indicated, united as a rule to 

 form the vascular bundles. They occur however, in many special cases, external to 

 and side by side with the latter in other regions also, and otherwise distributed. 



Scattered tracheides occur outside the vascular bundles, enclosed in other tissues 

 in the stems and scale-leaves of species of Salicornia and Nepenthes \ and in the 

 base of the leaf of the Isoetea;. 



In the many-layered, chlorophyll-containing parenchyma of the cortex of the 

 stem of those species of Salicornia which were examined, Duval-Jouve^ found, 

 according to the species, cylindrical or spindle-shaped tubes, which have exactly the 

 structure of air-containing tracheides. They are about as long as the chlorophyll- 

 containing layer of the cortex is thick, and have their longer axis perpendicular to 

 the epidermis, which they do not reach, but end one layer of cells further in, close 

 to one of the very numerous air-cavities of the stomata. Their other end is in 

 contact with the colourless inner parenchyma of the cortex, but does not extend 

 to a vascular bundle. In S. sarmentosa, patula ( = S. herbacea of most authors), 

 and fruticosa, the tracheides are rather regularly cylindrical-spindle-shaped ; their 

 completely colourless wall is thickened at the sides with a close and fine spiral 

 fibre, on the blunt ends it is smooth. In S. Emerici Duval-Jouve found the 

 tracheides but few, and weakly developed. In S. macrostachya they are irregularly 

 spindle-shaped, with lateral, short, pointed excrescences, and often with hooked ends ; 

 their membrane is uniformly and strongly thickened, it is smooth or scarcely pitted ; 

 they remind one to a certain extent of the rod-shaped sclerenchymatous cells in 

 the leaves of Proteaceae (comp. p. 130 and Chap. X). 



The tracheides of the species of Nepenthes ^, which are also filled with air, are 

 almost cylindrical, usually a little tapered at the ends, and of varying length, which 

 however hardly exceeds that of 10-20 cells of the parenchyma. Their colourless wall 

 has a close and delicate spiral thickening. They occur in the stem, distributed in all 

 parts of it, and in large quantity in the parenchyma ; similarly in the petiole and 



' [Cf. Mangin, Developpement des cellules spiralees. Ref. Bot. Centralbl. 1882, Bd. XII. p. 85.] 

 ^ Des Salicornia de I'llerault, Bulletin de la Societe Bot. de France, torn. XV. p. 140, pi. i. 

 ^ Korthals, Verhandelingen over de Xaturl. Geschied. d. Xederl. overzee. bezittingen ; Botanie, 

 p. I. — Compare also Unger, Grundlinien, p. 11. 



