HISTORICAL SKETCH 427 



"fluids" he evidently means subtle influences of undetermined nature. 

 With regard to the internal structure of plants, he says: " ... all that 

 we can find is, among the simplest, a cellular tissue without vessels but 

 variously modified and stretched or compressed according to the special 

 shape of the plant; and in the more complex, an assemblage of cells and 

 vascular tubes of various sizes, mostly with lateral pores, and a varying 

 number of fibers, resulting from the compression and hardening that a 

 portion of the vascular tube has undergone" (p. 235). 



As Gerould points out, Lamarck's cellular-tissue theory, like his 

 theory of evolution, was not supported by a body of well-authenticated 

 published facts. It was rather Mirbel (1808) who attempted to furnish 

 such observational data. In a work on Marchantia (] 831-1833) Mirbel 

 distinguished three modes of cell-formation: the formation of cells on 

 the surface of other cells, the formation of cells within older cells, and the 

 formation of cells between older cells. The first mode apparently 

 represented the budding of the germ tube arising from the spore, while 

 the second and third modes were formulated as the result of a misinter- 

 pretation of the process of cell multiplication in growing gemmae. Special 

 notice should be taken of the fact that by both men it was cellular tissue, 

 and not the individual cell, that was regarded as fundamental. Both 

 looked upon the organism as a cellular whole rather than an association 

 of elementary unicellular organisms. 



It was Dutrochet (1824) who first insisted upon the primary impor- 

 tance of the cells as individuals. This was largely because he was able 

 to resolve plant tissues by maceration into distinct cell units. Of the 

 cell he writes, "this astounding organ is truly the fundamental element of 

 organization; everything, indeed, in the organic tissues of plants is 

 evidently derived from the cell, and observation has just proved to us 

 that it is the same with animals." This is the essential point of the cell 

 theory; but before proceeding to a discussion of this theory we should call 

 attention to the discovery of the nucleus. 



Although nuclei were occasionally seen by earlier observers, the 

 major share of the credit for its discovery goes to Robert Brown (1773- 

 1858), for it was he who was first impressed by its probable importance 

 and emphasized it as a normal constituent of cells. Brown, a British 

 botanist celebrated chiefly for his taxonomic monographs and morpho- 

 logical researches, announced his discovery in a paper read before the 

 Linnsean Society in 1831. It was in leaf-cells of orchids that he saw the 

 nuclei most clearly. Concerning these observations he wrote as follows : 



In each cell of the epidermis of a great part of this family, especially of those 

 with membranaceous 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 ; it is not unfrequently 



