10 



THE CELL. AND PROTOPLASM 



de propagation, et destinees h former par ag- 

 glomeration 1 'individualite composee de tous les 

 vegetans dont 1 'organization de la masse coniporte 

 plus d 'une v4sicule. 



Here is clearly expressed the fact that 

 plant tissues are composed of cells, that 

 these cells are distinct individualities hav- 

 ing their own vital center of vegetation 

 and propagation and destined to form by 

 agglomeration the composite individuality 

 of all those plants whose organization is 

 composed of more than one cell — and all of 

 this twelve years before the publication of 

 "the epoch-making theory of Schleiden 

 and Schwann. ' ' 



In 1830, eight years before Schleiden 's 

 Beitrdge, the German botanist, Meyen 

 (1804^1810), in his Phytotomie wrote: 



Plant cells appear either singly so that each one 

 forms a single individual, as in some algae and 

 fungi, or they are united together in greater or 

 smaller masses, to constitute a more highly organ- 

 ized plant. Even in this case each cell forms an 

 independent isolated whole; it nourishes itself, 

 builds itself up, and elaborates raw nutrient ma- 

 terials, which it takes up, into very different sub- 

 stances and structures. 



He even spoke of such cells as "little 

 plants inside larger ones." Meyen also 

 described the circulating movement of cell 

 contents which had previously been ob- 

 served by Corti in 1774 and by Treviranus 

 in 1811. In his three- volume work on 

 Pflanzen-physiologie, published in 1837, 

 Meyen described cells as the "essential ele- 

 mentary organs of assimilation and con- 

 struction. ' ' 



Hugo von Mohl (1805-1872) is one of the 

 most important figures in the early develop- 

 ment of the cell theory. Like so many 

 of his contemporary biologists, he was a 

 doctor of medicine, a physiologist, and a 

 botanist, but his cell studies were chiefly 

 on plants. In 1831, seven years before 

 Schleiden, he announced that the cell is 

 the individual elementary unit of structure 

 in plants and that vessels and secreting 

 tubes are formed by end-to-end union of 

 elongated cells. His subsequent work on 

 cell division and on protoplasm is par- 

 ticularly important and will be referred to 

 later. 



The English botanist, Robert Brown 



(1773-1858), in 1831, discovered that clear 

 round bodies, which he called "areolae" or 

 "nuclei," are present very generally in 

 plant cells. He called attention to the fact 

 that nuclei had previously been seen and 

 figured in cells by Meyen, Purkinje, Brog- 

 niart, Braur, et al., but they had been re- 

 garded as unimportant. Brown recognized 

 nuclei as important organs of the cell and 

 his work marks a major stage in the de- 

 velopment of the cell theory. This discov- 

 ery is the more remarkable in that Brown 

 worked only with simple lenses and with- 

 out the aid of stains. About the same time 

 that Brown discovered and named the 

 nucleus, others, particularly Mirbel, saw it 

 in other objects. In 1836 Valentin (1810- 

 1883) observed it in the epithelial cells of 

 the conjunctiva and found a round cor- 

 puscle within it which he called the "nu- 

 cleolus, a kind of second nucleus within the 

 nucleus." 



Cell division had been seen in filamentous 

 algae by Turpin in 1826 and by Dumortier 

 in 1832. In 1835 Hugo von Mohl described 

 very exactly the manner of division of the 

 cells of the filamentous alga, Cladophora; 

 he found that a circular constriction ap- 

 pears around the middle of the cell and 

 this gradually deepens to form division 

 walls by which the cell is divided into two 

 daughter cells. Four years later he figured 

 and described the division of spore mother 

 cells in the scale moss, Anthosceros, some of 

 his figures (1839, Figs. 21-23) suggesting 

 that he had seen mitosis. Meyen in his 

 Neues Systems der Pflanzen-physiologie, 

 published in 1838, said that cell division is 

 everywhere easily and plainly seen in Con- 

 fervae, Chara, mycelia, and also in terminal 

 buds and root tips of Phanerogams. 



All of this significant work on cells pre- 

 ceded the famous publication one hundred 

 years ago by Matthias Schleiden (1804- 

 1881), Professor of Botany at Jena, entitled 

 Beitrdge zur Phytogenesis, which is gen- 

 erally credited with being the origin of 

 the cell theory. There is no doubt that 

 Schleiden was a stimulating figure and that 

 his polemics attracted more attention than 

 the modest research of some of his prede- 

 cessors, but so far from his being the 



