ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 77 



In commenting on this paper Hen* K. Goebel * points out that he 

 has already ascribed j to the aerial roots of Sonneratia and Avicennia the 

 property of serving as organs of respiration, their production being 

 incited by the peculiar habitat. 



Organs of Secretion.^ — Herr A. Tschirch has continued his in- 

 vestigations on the secretions and secreting structures of plants. (1) The 

 epidermal glands of Labiataa and Compositae, which contain ethereal oil, 

 are formed on two different types. In Labiates, wherever the glands 

 occur, they consist of a ring of secreting cells which lie beside one 

 another in fours or a multiple of four. The head-cell is divided by 

 radial partitions at right angles to the surface of the organ. In Com- 

 positae, on the other hand, the cells are arranged in layers one above 

 the other, often only the two upper layers secrete ; all the secreting 

 cells are divided by a median radial partition, usually at right angles 

 to the longitudinal axis of the organ. In the head-cell tangential walls 

 parallel to the surface are first formed, then a radial partition in each of 

 these divisions. From the surface the glands of Labiates exhibit a 

 central cell with a surrounding ring usually of eight, while those of 

 Compositaa form an elongated oval divided through the centre. 



(2) The origin of copaiva balsam is unique. The balsam is ex- 

 clusively formed in the wood, and there in the older portions. It arises 

 by retrogressive metamorphosis first of the walls of the vessels, 

 but implicating also the adjacent cells. Even in one-year twigs the 

 metamorphosis of some vessels was observed. Except in the case of 

 the very different " resin-gallen " of Conifers, this is the first certain 

 illustration of the possible modification of cellulose into resin or resin- 

 like substances. 



(3) In a second paper Herr Tschirch notes that the seat of the 

 cinchona-alkaloids is almost exclusively the cortical parenchyma, and 

 the contents of the cells. This cortical parenchyma is greatly increased 

 in the secondary cortex, while all the other elements of the bark dis- 

 appear. The increase in the alkaloid content depends chiefly on an in- 

 creased development of the thin-walled alkaloid-bearing tissue elements, 

 not on an increase of the absolute content of the individual cells. The 

 alkaloids pass only secondarily in the dry bark into the cell-walls. 



Anatomy of Water-plants. § — Dr. H. Schenck sums up the anatomical 

 characters of plants which grow entirely submerged in water. 



The leaf is almost always divided into capillary teeth, or is a narrow 

 grass-like ribbon ; exceptions are afforded by some species of Potamo- 

 geton. The parenchyma does not assume the spongy form with large 

 intercellular lacunae, the cells being prismatic in form and fitting closely 

 together without intercellular spaces, or else inclosing very large lacunae 

 in the interior; stomata are very rare, and the greater part of the 

 chlorophyll is contained in the epidermis. The vascular bundles are of 

 very simple structure, and are inclosed in a parenchymatous sheath, 

 which does not differ essentially in structure from the surrounding 



* Bot. Ztg., xlv. (1887) pp. 717-8. t See this Journal, 1887, p. 111. 



% Versamml. Deutsch. Naturf. u. Aerzte, Wiesbaden, Sept. 21, 18S7. See Biol 

 Centralbl., vii. (1887) p. 133. 



§ Ulilworm und Haenlein's Biblioth. Bot., i. (1886) pp. 1-67 (10 pis.). Cf. this 

 Journal, 1886, p. 272. 



