SCHLEIDEN 171 



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 character- 

 istic spot which he called the areola or nucleus. 1 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 (Kcriic/iett), 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. Cell-development always takes place 

 within existing cells, and either one or many new cells may 

 be formed within the mother-cell. Schleiden's views on cell- 

 formation were drawn from some rather imperfect observa- 

 tions on the embryo-sac and pollen-tube, but he extended his 

 theory to cell-formation in general. Though wrong in almost 

 all respects the theory had at least the merit of fixing 

 attention upon the really important constituents of the cell, 

 the nucleus and the cell-plasma. To Schleiden, too, we owe 

 the conception of the cell as a more or less independent living 

 unity, whose life is not entirely identified with the life 

 of the plant as a whole. " Each cell," he writes, " carries on 

 a double life ; one a quite independent and self-contained 

 1 Trans. Linnean Soc., xvi., p. 710, 1833. 



