Page:History of botany (Sachs; Garnsey).djvu/348

328 bears. Meanwhile Nägeli had also distinguished the protoplasm from everything else in the cell, and noticed its pre-eminent importance in cell-formation and its nitrogenous character.

We must not omit to mention here, that investigations into the processes of cell-formation compelled observers to search for the spots where cell-formation actually takes place, and thus the fact was ascertained, that cells in statu nascendi are not to be found in all parts, not even in all growing parts of the plant, but that we must look for them in the so-called puncta vegetationis in the stem and root, in the youngest lateral organs, and between the bark and the wood in woody plants. About this time a new idea began to be attached to the word cambium, which Mirbel had used in the sense of a nourishing juice saturating the plant; it was now applied to the tissue-masses in which the formation of new cells takes place, and specially to the very thin layer of tissue lying between the wood and the rind, from which new layers of wood and rind in woody plants are formed a layer, which according to Mirbel's theory had been a mass of sappy matter, in which new cells arise as vacuoles.

Unger in an enquiry into the growth of internodes ('Botanische Zeitung,' 1844) again declared himself as an opponent of Schleiden's theory. He maintained first of all and erroneously that the cell-nucleus is not of general occurrence in tissue where division is taking place, but he argued rightly from the position of the cells, from the difference of thickness in their walls, and from their relative size, in favour of their multiplication by the formation of dividing walls; he noticed the part played by the cell-contents in the multiplication of cells in hairs, and asserted that merismatic cell-formation (cell-division) is the general rule in the growth of organs of vegetation, while he distinctly declared that it was not possible to bring all that is actually seen at the spots where formation of cellular tissue is taking place into agreement with Schleiden's theory. But Unger did not observe the processes that take place in cell-