ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 607 



(4) Structure of Organs. 



Roots of Araceae.* — Herr M. Lierau lias examined the structure of 

 the roots in 46 genera and about 130 species of Araceae. 



The epidermis usually consists of a single layer in the terrestrial 

 species ; occasionally there are two, three, or even four layers of cells. 

 The root-hairs are developed from the outer layer, and are either simj)le 

 or forked, but never thickened like those of many epiphytic orchids. In 

 the epiphytic species the epidermis is replaced by a root-sheath or 

 velamen, usually thin-walled, but consisting, in some species of Antliu- 

 rium, of several layers of thickened tracheides. The velamen is often 

 only a temporary structure. It is always separated from the cortical 

 parenchyma by a protecting sheath or outer endoderm, easily recognized 

 by the fine striation of its suberized walls on longitudinal section, and 

 the wavy appearance of the walls on tangential section. This outer 

 endoderm is very commonly a phellogen-layer, passing over later into 

 cork ; in some genera it disajipears altogether with the velamen. 



The cortex of the root is more or less permeated by large air- 

 passages in the aquatic or paludose species. Almost all possess raphides, 

 and bundles of crystals are found in some cases. Keceptacles both for 

 secretion and excretion abound, such as the oil-receptacles in Acorus, the 

 tannin-cells in Anihurium, latex-vessels, and resin-passages. Spicular 

 cells in the intercellular spaces occur only in the Monsteroidefe. An 

 inner endoderm was found in the root of all Aracese examined, and is 

 usually somewhat suberized. The axile fibrovascular bundle is almost 

 always of typical structure. 



Mechanical Protection of Bulbs. f — Herr F. v. Tavel describes the 

 mode in which the tissue of bulbs which is used as a store-house for 

 reserve food-material is protected against pressure or impact throughout, 

 in species belonging to the genera Grinum, Brunsvigia, Allium, Gagea, 

 and Narcissus. These contrivances have no relation with the systematic 

 position of the species. They consist in most instances of stereides, 

 which may be collected into a sclerenchymatous layer of cells. Their 

 special development depends on the nature of the climate, and of the 

 injurious influences against which the particular bulb has to be 

 protected. 



Floating-roots of Sesbania aculeata.J — Dr. D. H. Scott gives the 

 following as the results of his examination of the floating-roots of 

 Sesbania aculeata Pers., a plant belonging to the papilionaceous tribe 

 Galegeae. (1) The floating tissue of the roots of Sesbania is a secondary 

 cortical structure, arising from a phellogen. (2) This tissue, though 

 falling under the definition of periderm, differs from cork in its per- 

 manently living cells, its non- suberized cell- walls, and its large inter- 

 cellular spaces, in which alone air is contained. In all these respects it 

 agrees with the floating-tissue of the stem of Neptunia oleracea. (8) The 

 phellogen originates immediately outside the endoderm, thus differing 

 from the phellogen of most roots with typical periderm. 



* Lierau, M., 'Beitr. z. Kenntniss d. Wurzeln d. Araceen,' 37 pp. and 1 pi., 

 Breslau, 1887. See Bot. Oentralbl., xxxiv. (1888) p. 53. 

 t Ber. Deutsch. Bot. GeselL, v. (1S87) pp. 438-58 (2 pis.). 

 X Ann. of Bot., 1. (1888) pp. 307-13 (1 pL). 



