ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 529 



to the natural cylindrical or prismatic cells in tlie interior of the 

 plant. The only instance of a different behaviour is in an artificial 

 cylinder of gum-tragacanth, which resembles the stem-cell of 

 Caulerpa. All the more delicate points of structure, such as pitting 

 of the membranes, dots, or pores on the one hand, or projecting 

 masses, ridges, or spiral bands on the other hand, can be imitated 

 artificially in colloids rendered rigid. The former can be referred to 

 the phenomena of hollow spheres, the latter to local compression or 

 dilatation of the rigid mass. 



Origin of " Cell-passages."* — In the annual rings of many woody 

 plants there are visible even to the naked eye long crescent-shaped 

 spots, which are seen on tangential section to be passages, and which 

 have been described under various names, " Markflecke," " Zell- 

 gange," &c. These have been examined in a number of trees, willow, 

 birch, alder, Pyrus, &c., by M. Kienitz, who has determined them to 

 be always abnormal structures ; and in fact to be passages produced 

 by larvas, and filled up by new cells, the larvae having devoured the 

 cambium and other young cells at the time when the ring was being 

 formed. The cells which break through the margin of the wound 

 grow rapidly, and divide further by delicate septa ; at the same time 

 the cambial ring closes Tip completely ; and from this time normal 

 wood and normal cortex are again formed above the sui-face of the 

 wound ; while, quite independently of the new cambium, the cavity is 

 closed by the growth of the cells. 



Collenchyma.t — In a monograph on this subject, E. Giltay treats 

 chiefly of the mechanical importance and properties of collenchyma, 



CoUenchyma occurs either at the periphery of organs, or less often 

 in the centre ; in the form either of collenchymatous bast, distinguished 

 by the secondary sclerenchymatous elements not being lignified, as in 

 Polemonium reptans, Lycium harhariim, Pe2:)eromia, JBoti-ycMum Lunariat 

 and Ophioglossum vulgatum, or as collenchymatously thickened medul- 

 lary cells, as in Panicum imhecille and Erythrina marmorata. In the 

 collenchyma of the vascular bundles the author includes not only the 

 ridges in Umbelliferse, Labiatse, &c.j but also the stereome of Aroidese, 

 inaccurately termed sclerenchyma by Schwendener. 



Although collenchyma occurs comparatively rarely in mono- 

 cotyledons, it was found in all the climbing species examined, Aspa- 

 ragus scandens, Lapageria rosea, Smilax, and PoxburgMa viridiflora. 



With regard to the power of swelling of collenchyma, the author 

 shows by numerous measurements on various plants, Fceniculum 

 vulgare, Dipsacus ferox, Achillea Jilipendula, Pyrethrum multiflorum, 

 and Pubia tindorum, that the wall of collenchymatous cells may be 

 32 per cent, thicker in water than in alcohol of 95 per cent. 



As regards the history of its development, Giltay agrees with 

 Haberlandt that the origin of mechanical tissue is as various as 

 possible. 



* Bot, Centialbl., xiv. (1883) pp. 21-6, 56-61 (2 pis.). 



t Giltay, E., ' Het Collenchym,' 186 pp. (5 pis.) Leyden, 1882. See Bot. 

 Centralbl., xiii. (1883) p. 409. Cf. also this Journal, 1. (1881) p. 768; ii. (1882) 

 pp. 71, 812. 



Ser. 2.— Vol. III. 2 M 



