two elaborate treatises on the minute structure of plants, which had been quite independently worked out, the one by Grew, an Englishman, the other by Malpighi, an Italian. But their admirable work remained for more than a hundred years the standard of knowledge in plant anatomy, while the overwhelming authority and example of the Linnæan school reduced botany to the superficial examination of dried fragments, and comprised all needed knowledge of a plant in the determination of its Latin name.
For the beginnings of our real knowledge of the cell we must, then, leap the chasm of a century and a half to come again upon a period of improvement of the microscope as affording the means for further advance. In this case the important discoveries of Amici, resulting in lens systems in which both chromatic and spherical aberration were largely corrected, offered to histologists far better tools than had hitherto been at their command. Grew and Malpighi had distinguished in plants two kinds of elements, approximately isodiametric cells and much elongated vessels; but Treviranus had shown, early in the present century, that vessels are derived from rows of cells by the obliteration of intervening walls. And about the same time he had rediscovered the now familiar circulation in the gigantic cells of the brittleworts, or Characeæ, which, first described in 1772 by Corti, had been forgotten in the zeal for taxonomy which possessed his contemporaries. But gradually the idea grew that the contents within the walls of the cell are of importance; that the real cell is a living thing which nourishes itself and grows. One of the earliest clear expressions of this idea was written by Meyen in 1830, who spoke of cells of higher plants as "little plantlets in the greater." But the general spread of such views and general interest in cell problems date from the time when Schleiden's clear mind was turned upon them. Going directly lo the heart of the whole matter as it then stood, he asked, "How do cells arise?" and set himself to answer the question. As a probable clew to its solution he seized, by a happy inspiration, upon the discovery made a few years before by Robert Brown that the cells of certain plants studied by him contained each a rounded, rather highly refractive body, which he had called, and which is still called, the nucleus. Demonstrating, with the aid of others, the very general occurrence of this structure in plant cells, Schleiden made it the center of his theories of cell life and cell formation. It is true that his belief that new nuclei are formed by a sort of crystallization out of a mother-liquor and then form centers for the formation of new cells, has been proved to be incorrect. And it is equally true that his idea of the cell as a closed chamber filled with fluid, whose wall is its most essential part, is no longer entertained.