THE ANNALS AND MAGAZINE OF NATURAL HISTORY, [THIRD SERIES.] No. 12. DECEMBER 1858. XL. — On the Cambium-layer of the Stem of the Phanerogamia, and on its Relation to the Increase of Thickness. By H. von Mohl*. During the last twenty or thirty years, investigations on the development of the stem have led to the discovery that, in spite of the great difference of structure in the stems of the Monocotyledons and Dicotyledons, the course of their develop-ment presents a far greater agreement than was formerly ima-gined. Satisfactory as this progress is on the one hand, yet, on the other, labourers in this field have, at least so it appears to me, promulgated many erroneous theories ; hence a discussion of this subject will not be inopportune. It will be most convenient to recur, in the first place, to Schleiden's works. In his explanation of the peculiarities of vegetable tissues f, he assumed the existence of three stages of cell-development in the earliest period. In the first stage, the new products present themselves in the form of an apparently structureless, yellow, pultaceous mass; in the second stage, in which the process of cell-formation has just ceased, there is a distinct delicate cellular tissue with more homogeneous con-tents, which, however, is still completely saturated with sap ; in the third stage, the cellular tissue assumes a blackish appearance, arising from the fact that all the intercellular passages are then emptied of sap and contain only air. According to Schleiden's view, the arrangement of the cel-lular tissue influences the conformation of the stem exclusively in the first stage. This depends — 1. on the arrangement of the * Translated from the ' Bot. Zeitung,' xvi. p. 184 et seq., June 25 and July 2, 1858, by Arthur Henfrev, F.R.S., &c. t Grundz. d. wiss. Bot. 1843, ii. 127. Ann. §â– Mag. N. Hist. Ser. 3. Vol. ii. 27
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