CHAP, in.] of Ccll-incmbrane in Plants. 299 



of the solid framework of plants, because the historical signifi- 

 cance of his investigations into the history of development can 

 only be understood in connection with the questions to be 

 treated in the following chapter. But we shall not limit our- 

 selves to publications which appeared before 1845, though we 

 may be thus compelled to notice researches which in succession 

 of time belong to the next period, and indeed almost to the 

 present moment. 



i. The view that the cell is the sole and fundamental 

 element in vegetable structure had been already maintained 

 by Sprengel and Mirbel, but not supported by exact observa- 

 tions. Treviranus too had shown that the vessels in wood 

 are formed by the union of rows of cell-like tubes, but he had 

 never arrived at a thoroughly clear conception of the matter. 

 On the one side was the theory that the plant consists entirely 

 of cells, on the other, and for long the old and strange view, 

 that the spiral thread was an independent elementary organ of 

 vegetable structure, a view which Meyen still maintained in 

 1830. Von Mohl must be regarded as the first who took up the 

 all-important position, that not only the fibrous elements of 

 bast and wood, which had long been considered to be elon- 

 gated cells, but the vessels of the wood also are formed from 

 cells ; and we may on this point give great weight to his own 

 assertion that he was the first who observed the formation of 

 vessels from rows of closed cells. This discovery happened 

 in the year 1831, and he describes distinctly, though briefly, 

 the decisive observations in his treatise on the structure of the 

 palm-stem. At the points of constriction in the vessels he saw 

 the dividing walls, the existence of which had been denied by 

 all former phytotomists ; ' these dividing walls,' he says, ' are 

 entirely different from the rest of the membranes of the plant, 

 being formed of a network of thick fibres with openings 

 between them.' He studied the history of the development 

 of these vessels both in palms and in dicotyledonous plants. 

 ' In the young shoot,' he says, 'are found at the spots, where 



