V. ANATOMY AND PHYSIOLOGY IN THE NINETEENTH CENTURY

"I shall conclude my observations on Orchideae," said Brown, "with a notice of some points of their general structure, which chiefly relate to the cellular tissue. In each cell of the epidermis of a great part of this family, especially of those with membranous leaves, a single circular areola, generally somewhat more opaque than, the membrane of the cell, is observable. This areola, which is more or less distinctly granular, is slightly convex, and although it seems to be on the surface is in reality covered by the outer lamina of the cell. There is no regularity as to its place in the cell; it is not unfrequently, however, central or nearly so.

"As only one areola belongs to each cell, and as in many cases where it exists in the common cells of the epidermis, it is also visible in the cutaneous glands or stomata, and in these is always double—one being on each side of the limb—it is highly probable that the cutaneous gland is in all cases composed of two cells of peculiar form, the line of union being the longitudinal axis of the disk or pore.

"This areola, or nucleus of the cell as perhaps it might be termed, is not confined to the epidermis, being also found, not only in the pubescence of the surface, particularly when jointed, as in cypripedium, but in many cases in the parenchyma or internal cells of the tissue, especially when these are free from the deposition of granular matter.

"In the compressed cells of the epidermis the nucleus is in a corresponding degree flattened; but in the internal tissue it is often nearly spherical, more or less firmly adhering to one of the walls, and projecting into the cavity of the cell. In this state it may not unfrequently be found. in the substance of the column and in that of the perianthium.

"The nucleus is manifest also in the tissue of the stigma, where in accordance with the compression of the utriculi, it has an intermediate form, being neither so much flattened as in the epidermis nor so convex as it is in the internal tissue of the column.

"I may here remark that I am acquainted with one case of apparent exception to the nucleus being solitary in each utriculus or cell—namely, in Bletia Tankervilliae. In the utriculi of the stigma of this plant, I have generally, though not always, found a second areola apparently on the surface, and composed of much larger granules than the ordinary nucleus, which is formed of very minute granular matter, and seems to be deep seated.

"Mr. Bauer has represented the tissue of the stigma, in the species of Bletia, both before and, as he believes, after impregnation; and in the latter state the utriculi are marked with from one to three areolae of similar appearance.

"The nucleus may even be supposed to exist in the pollen of this family. In the early stages of its formation, at least a minute areola is of ten visible in the simple grain, and in each of the constituent parts of cells of the compound grain. But these areolae may perhaps rather be considered as merely the points of production of the tubes.

"This nucleus of the cell is not confined to orchideae, but is equally manifest in many other monocotyledonous families; and I have even found it, hitherto however in very few cases, in the epidermis of dicotyledonous plants; though in this primary division it may perhaps be said to exist in the early stages of development of the pollen. Among monocotyledons, the orders in which it is most remarkable are Liliaceae, Hemerocallideae, Asphodeleae, Irideae, and Commelineae.

"In some plants belonging to this last-mentioned family, especially in Tradascantia virginica, and several nearly related species, it is uncommonly distinct, not in the epidermis and in the jointed hairs of the filaments, but in the tissue of the stigma, in the cells of the ovulum even before impregnation, and in all the stages of formation of the grains of pollen, the evolution of which is so remarkable in tradascantia.

"The few indications of the presence of this nucleus, or areola, that I have hitherto met with in the publications of botanists are chiefly in some figures of epidermis, in the recent works of Meyen and Purkinje, and in one case, in M. Adolphe Broigniart's memoir on the structure of leaves. But so little importance seems to be attached to it that the appearance is not always referred to in the explanations of the figures in which it is represented. Mr. Bauer, however, who has also figured it in the utriculi of the stigma of Bletia Tankervilliae has more particularly noticed it, and seems to consider it as only visible after impregnation."[2]

SCHLEIDEN AND SCHWANN AND THE CELL THEORY

That this newly recognized structure must be important in the economy of the cell was recognized by Brown himself, and by the celebrated German Meyen, who dealt with it in his work on vegetable physiology, published not long afterwards; but it remained for another German, the professor of botany in the University of Jena, Dr. M. J. Schleiden, to bring the nucleus to popular attention, and to assert its all-importance in the economy of the cell.