EXTRAFASCICULAR SIEVE-TUBES. 23 1 



Acad. Math, naturw. Classe, Bd. 24, p. 179 (1864). — Ueber kugelformige Zellverdick- 

 ungen in der Wurzelhiille einiger Orchideen, Sitzgsbr. d, Wiener Acad. Bd. 49. — Ueber 

 Hartvvegia comosa, &c., ibid. Bd. 49, p. 138. — Also Nicolai, das Wachsthum der Wurzel. 

 Schr. d. Physik. Gesellsch. z. Konigsberg, VII. (1865) p, 66. The remarkable white 

 ' parchment-like ' skin of the Orchids has been known since Link (Elem. philosoph. bot, 

 Ed. I. (1824) p. 395), and repeatedly investigated by Meyen (Phytotomie, p. 163 ; Physi- 

 ologic, p. 47) : Mohl, Unger (Anatomic u. Physiol, p. 194) in the Orchids, and by Schleiden 

 (Grundziige, Ed. 3, p. 284) in these and the Aroids: but a clear view of the case was 

 not obtained, since according to Meyen and Schleiden the ehdodermis was taken for the 

 epidermis (its short cells were regarded by Schleiden as stomata). Schacht (Lehrb. I. 

 p. 258) and Oudemans regarded only the simple, outermost air-containing layer as the 

 epidermis, and the inner layers as an hypodermal ' intermediate ' tissue. The statements 

 of Chatin, Anatomie des plantes aeriennes de I'ordre des Orchidees, Mem. Soc. de Cher- 

 bourg, Vol. IV, 1856, and of Fockens, Ueber die Luftwurzeln, &c., Diss. Gottingen, 

 1857, have been corrected by Leitgeb and Oudemans in those points in which they 

 differ from the above description. 



Sect. 57. Sieve-tubes occur outside the vascular bundles in a relatively large 

 number of stems of Dicotyledons, and of some Monocotyledons : they form small 

 groups or bundles, which traverse the parts longitudinally, and anastomose at the 

 nodes not only with one another, but also with those of the vascular bundles. The 

 tubes are always accompanied by the same delicate, elongated cells, as in the vascular 

 bundles — these will be described when the latter are treated of — often also by 

 sclerenchymatous fibres and milk-tubes. 



Many Dicotyledons have bundles of sieve-tubes at the periphery of the pith, near 

 to the ring of vascular bundles, many Melastomaceae also have them scattered through 

 the pith. In many plants — Myrtaceae, Daphne, Strychnos, Apocynaceie, and 

 Asclepiadaceae, Convolvulacese, often also in the, families to be named below — 

 they approach so closely to the inner margin of- the vascular bundles that they are 

 better to be regarded as parts of these, and in all cases the bundles of the pith are 

 related so closely and in such various ways to the system of vascular bundles that the 

 subject will be returned to when the latter is described : reference may therefore be 

 made to Sects. 62 and 103. Therefore we need only mention here the bundles of 

 sieve-tubes which separately traverse the periphery of the pith in the stems of species 

 of Solanum (S. tuberosum, Dulcamara), Nicotiana, Datura, and Cestrum ; of many 

 Campanulaceae, as Campanula cervicaria, lamiifolia, glomerata, and pyramidalis, but 

 not C. INIedium or rapunculoides ; further, those bundles, accompanied by milk- 

 tubes, which are found in the same position in the Cynaraceous plant, Gundelia 

 Tournefortii, and those which occur in many Cichoriaceae of the genera Lactuca, 

 Scorzonera, Sonchus, Tragopogon, Hieracium, but not in Chondrilla, Taraxacum, 

 and Apargia. In Cichorium the bundles of sieve-tubes are absent in the stem, 

 but appear in the petiole near to the vascular bundles '. 



In the cortical parenchyma outside the ring of vascular bundles sieve-tubes are 

 of constant occurrence in thick Cucurbitaceous stems ^ (Cucurbita, Lagenaria, 

 Cucumis, Ecbalium). Here they lie close to the inner limit of the intra-cortical ring 



1 Hanstein, Die Milchsaftgefasse, pp. 57, 68, &c.— Trecul, Comptes Rendus, 27 Nov. 1865. 

 ^ Sanio, Botan. Zeitg. 1864, p. 227. 



