of the solid framework of plants, because the historical significance 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.
1. 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 observations. 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
