CHAPTER V. 

 SIEVE-TUBES. 



Sect. 43. The Sieve-tubes, Tubi cribrosi, were first clearly distinguished by 

 Th. Hartio-^, in the year 1837, as essential constituents of the bast and of the 

 vascular bundles of Phanerogams, and were in some cases designated by the 

 above name, while in others they were termed sieve- fibres. After lying unrecognised 

 for many years, Hartig's observations were confirmed and extended, especially by 

 Mohl, Nageli, and Hanstein ^ 



The chief points of occurrence of the organs in question are those above 

 mentioned j they are rarely found elsewhere. They are present in both Phanerogams 

 and Ferns. They have been most thoroughly investigated in the Angiosperms. They 

 may therefore be treated of first as they occur in the latter plants, and afterwards the 

 peculiarities to be found in the other divisions may be added. 



The articulation of the sieve-tubes in the plants in question is throughout 

 similar to that of the vessels treated in the preceding chapter. They arise from 

 longitudinal rows of elongated, cylindrical, or prismatic cells, and these remain 

 always clearly distinguishable and separable as their members. On the faces with 

 which the members are mutually contiguous they come into open comraunicatioii 

 through the sieve-plates or sieve-fields, which are circumscribed portions of the wall, 

 by means of numerous very small, perforated pits, the pores of the sieve. 



The form of the members of the tubes is that above stated. Their ends are 

 limited by one flat, or slightly concave wall (concave on the under side) ; and this is 

 either almost horizontal, or at. most slightly oblique, and in that case as a rule slightly 

 broader than the middle of the member ; or it is very strongly inclined, and cuts the 

 lateral-wall on one side at a very acute angle, so that each end of a member is 

 bevelled on one side like a chisel. The inclination of the terminal surfaces is in 

 the latter case — though not invariably and exactly — towards the radial plane. 



^ Vergl. Untersuchungen iiber die Organisation des Stammes d. einheim. Waldbaiime; in 

 Jahresber. iib. d. Fortschritte d. Forstwlssenschaft, &c. p. 1 25.-;— Compare further Hartig, Vollst, 

 Naturgesch. d. forstl. Culturpfl. Berlin, 1851 ; Botan. Zeitg. 1853, p. 571.— Ibid. 1854, p. 51. 



" Von Mohl, Einige Andeutungen iiber d. Bau d. Bastes, Botan. Zeitg. 1855, p. 865. — Nageli, 

 Ueber d. Siebrohren, Sitzsber. d. Miinchener Acad. Feb. 1861. — Hanstein, Die Milchsaftgefasse u, 

 verw. Organe, &c. Berl. 1864. — Mohl calls the members of sieve-tubes ' latticed cells' (Gitterzellen). 

 P. Moldenhawer had already distinguished them in part as ' vasa propria^ but confused them with 

 other elements under this name. [See further Wilhelm, Beitrage z. Kenntniss d. Siebrohren-appaiates 

 Dicotyler Pflanzen, Leipzig, 1880. — Janczewski, Sur les tubes cribreux, Mem. Soc. Cherbourg. 1881. 

 — Idem, Et. Comp. sur les tubes cribreux, Ann. Sci. Nat. 6 ser. torn. XIV. 1882. — Russow, Sur la 

 structure et le developpement des tubes cribreux, Ann. Sci. Nat. 6 ser. torn. XIV. 1882.] 



