ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
211 
applied to such bundles, ought to bo disused, from tlio point of view of 
the origin of the internal phloem. 
Formation of Duramen.* — According to Herr K. v. Tubeuf the excre- 
tion of gum or the formation of thyllae in the vessels of dicotyledonous 
trees affords no analogy to the excretion of resin in Conifers. The 
purpose of the formation of duramen as a consequence of injury to the 
stem is the prevention in the interior of the plant of differences 
in air-pressure, in the proportion of oxygen, and in that of moisture, 
between the air within the plant and the external atmosphere. The 
substances which cause the hardening always arise from living cells, and 
the walls of the duramen are always impregnated with tannin. 
Medullary Rays.f — Herr L. Kny has investigated the histological 
structure of the medullary rays of dicotyledonous woody plants. He 
finds them to be composed essentially, in the majority of cases, of two 
kinds of cell, which he calls medullary palisade-cells and medullary 
merenchyme-cells, and describes in detail their character in the case of 
Salix fragilis. .The former are usually greatly elongated in their longi- 
tudinal diameter, and lie close together without intercellular spaces ; 
the latter are usually elongated radially, and have narrow intercellular 
spaces lying transversely between their layers. In the case described, 
the innermost portion of the rays which lies in the region of the spiral 
vessels consists exclusively of palisade-cells. The two kinds of cell differ 
also in the mode of their punctation. The walls of the merenchyme- 
cells which are in contact with vessels are destitute of pits, while pits 
occur abundantly on their upper and under walls. Where, on the other 
hand, palisade-cells lie in contact with vessels, the intervening walls 
are provided with large polygonal slightly bordered pits, which are 
wanting in the palisade-cells of other parts of the medullary rays. Even 
in later rings belonging to branches several years old the merenchyme- 
cells are sometimes entirely wanting. Medullary rays consisting exclu- 
sively of merenchyme-cells were never seen by the author. 
Function of the Sieve-portion of Vascular Bundles. :f — Dr. J. Blass 
adduces further arguments in favour of his view that the chief function 
of the sieve-tubes is the supply of food-material to the wood-elements 
and to the formative cambium. In many trees — Tilia, Quercus, Syringa , 
Fraxinus, Pojpulus, Betula — as well as in herbaceous plants, he finds the 
sieve-cells in the immediate proximity of the cambium. By ringing 
the stems of both woody and herbaceous plants, it can be shown that no 
copious flow of albuminoids takes place out of the sieve-tubes. 
M. H. Lecomte§ criticizes very unfavourably Dr. Blass’s theory 
that the sieve-tubes are the locality of the formation, and not merely 
the conducting tissue, for albuminoid substances. He asserts that the 
theory rests on assumptions rather than on proved facts, and points out 
that it is opposed to other facts which have been established with regard 
to these vessels. 
M. Lecomte further states that Dr. Blass does not pay sufficient 
* Zeitschr. f. Forst- u. Jagdwesen, 1889, pp. 385-403. See Bot. Centralbl., xliv. 
(1890) p. 232. f Ber. Deutsch. Bot. Gesell., viii. (1890) pp. 176-88 (1 pi.). 
X Jahrb. f. Wiss. Bot. (Pringsheim), xxii. (1890) pp. 253-92. Cf. this Journal, 
1890, p. 622. § Journ. de Bot. (Morot), iv. (1890) pp. 299-300, 400-4. 
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