In the early 19th century, interest in plant anatomy revived somewhat, and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von Mohl.[239] As a result of their work the fact was established that the tissues of plants are composed of elements which can, with few exceptions, be reduced to one simple fundamental form--the spherical closed cell. Thus the vessels of plants are formed by coalescence of cells, fibres by the elongation of cells and the thickening and toughening of their walls. At this time, interest was concentrated on the cell-wall, to the almost total neglect of the cell-contents; the "matured framework" of plant cells, to use Sach's convenient phrase, was the chief, almost the sole, object of study. And it was natural enough that the mere architecture of the plant should monopolise interest, that the composition of the tissues out of the cells, and the fitting together of the tissues to form the plant should awaken and hold the curiosity of the investigator; even the modifications of the cell-walls themselves, their rings and spiral thickenings and pits, offered a fascinating field of enquiry. The idea that the cell-contents might show a characteristic and individual structure had hardly dawned upon botanists when Schleiden published his famous paper, _Beitraege zur Phytogenesis_.[240] Schleiden's theme in this paper is the origin and development of the plant cell, a subject then very obscure, in spite of pioneer work by Mirbel. A few years before, Robert Brown had called attention to the presence in the epidermal cells of orchids and other plants of a characteristic spot which he called the areola or nucleus.[241] Schleiden saw the importance of this discovery, confirmed the constant presence of the nucleus in young cells, and held it to be an elementary organ of the cell. He named it the cytoblast because, in his opinion, it formed the cell. It was embedded in a peculiar gummy substance, the cytoblastem, which formed a lining to the cellulose cell-wall. Within the nucleus there was often a small dark spot or sphere--the nucleolus. The nucleus, Schleiden thought, originated as a minute granule in the cytoblastem which gradually increased in size, becoming first a nucleolus (_Kernchen_), and then, by further condensation of matter round it, a nucleus. Several nuclei might be formed in this way in a single cell. New cells took their origin directly from a full-grown nucleus, in a peculiar way which Schleiden describes as follows:--"As soon as the cytoblasts have reached their full size a delicate transparent vesicle arises on their surface; this is the young cell, which at first takes the shape of a very flat segment of a sphere, of which the plane surface is formed by the cytoblast, the convex side by the young cell itself, which lies upon the cytoblast like a watch-glass on a watch" (p. 145). The young cells increase in size and fill up the cavity of the old cell, which is in time resorbed.