ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
501 
author proposes the term “ sieve-xylem ” for these groups of sieve- 
cells in the xylem of Dicella , “ phloem-islands ” for those of StrycJinos. 
Obliteration of Sieve-tubes.* — According to Herr H. Bliesenick, 
the obliteration of sieve-tubes in the secondary cortex is always due to 
the pressure of mechanical elements. The partial obliteration of the 
tubes in the autumn by the formation of callus is always again neu- 
tralized when the sap begins to flow in the spring ; the sieve-pores are 
quite open at the time of expansion of the first leaves. Y\ hen the sieve- 
elements are closed, the tissue sometimes assumes a horny consistency, 
forming what the author terms ceratenchyme. 
According to the mode in which the obliteration takes place, the 
author distinguishes four types in dicotyledonous trees, viz. : — (1) The 
bast-fibres are arranged in radial rows in such a way that the sieve- 
elements lying between them are only slightly obliterated, or not at all. 
(2) The bast is arranged in tangential rows of variable strength, and is 
often accompanied by sclerenchymatous cells ; the bast-fibres take various 
forms for the purpose of protecting the sieve-elements ; ceratenchyme is 
usually formed, with obliteration of elements. (3) The bast-fibres are 
very weak or entirely wanting ; there is a greater or less development of 
sclerenchymatous cells. (4) Neither bast-fibres nor sclerenchymatous 
cells are formed in the secondary cortex. A number of illustrations are 
given of each type. 
Formation of Cork.j — Herr J. E. Weiss designates as jphelloid-cells 
those which are separated on the outer side from the phellogen, but 
which are not themselves suberized. The formation of such cells is 
frequent, but is generally characteristic of particular families or sections. 
Their formation within cork can only take place when the initial cells of 
the cork are imbedded deep in the primary or secondary cortex. The 
various modes in which this may occur are described in detail. The 
cork-cells may subsequently divide again by a radial wall. In many 
plants the cork-cells are indistinguishable from the cells of the pro- 
tecting sheath, and undergo at a later period similar changes. When 
lying isolated between two cork-cells, there are often intercellular 
passages. Cork is frequently formed even in annual plants. Its cells 
are always formed in centripetal succession, while the cells of the 
phelloderm are always formed centrifu gaily. 
The value of the mode of formation of cork for systematic purposes 
is then discussed in detail, the most important point being the region 
where the cork begins to be formed. 
Histological Structure of Carex.J — Herr A. Mazel has examined 
the structure of forty-three species of Carex , especially on two points, the 
stomates, and the vascular bundles of the rhizome and root. He finds 
the differences to correspond with the habit of the species rather than 
with characters which ,can be used in classification. The stomates are 
* 1 Ueb. d. Obliteration d. Siebrohren,’ Erlangen, 1891. See Bot. Centralbl., xlix. 
(1892) p. 274. 
t Denkschr. K. Bayer. Bot. Gesell. Begensburg, vi. (1890) 69 pp. and 1 pi. See 
Bot. Centralbl., 1. (1892) p. 88. 
f ‘ Etudes d’anat. comp. s. 1. organesde vegetation dans le genre Carex? Geneve, 
189i, 213 pp. and 7 pis. See Bot. Centralbl., 1891, Beih., p. 514. 
