While men of the highest powers of mind were devoting themselves to the solution of these difficult problems, the study of tissues was not neglected in the years after 1840, and here too it was Nägeli who gave the chief impulse and the direction to further development. In the periodical which he published in conjunction with Schleiden he had already (1844-46) given an account of some searching enquiries which he had made into the first processes in the formation of vascular bundles from uniform fundamental tissue; in the Cryptogams he observed the production of the tissue of the whole plant from the apical cell of the growing stem, and this discovery, still further pursued by Hofmeister especially, has given rise during the last twenty years to a copious literature, which has been of service to the theory of the formation of tissues, to morphology, and consequently also to systematic botany. The researches of Hofmeister, Nägeli, Hanstein, Sanio, and others into the first formation of vascular bundles from the fundamental tissue of young organs led to important results for morphology, in so far as it was now for the first time possible to judge of the morphological value of anatomical and histological relations. The growth in thickness of woody plants, a question of primary importance to vegetable physiology, was first made intelligible by the discovery of the mode of formation of vascular bundles and their true relation to cambium; Hanstein and Nägeli, and afterwards Sanio especially, cleared up the questions connected with growth in thickness in their main features before and after 1860. * * * * * When we pass on to show how the great results above-mentioned were attained, we encounter some difficulties. After 1840 botanical literature multiplied to an extent before unknown; it is from elaborate monographs on single subjects in phytotomy, from some text-books, and especially from smaller essays in botanical periodicals that we must gather an account of the further development of scientific thought. Much as the founding of scientific periodicals has facilitated communication between professed botanists, yet this form of literature makes it more difficult to see the way clearly through the work of earlier periods and to discover the historical connection in the science, not to speak of the harm that usually results from it to young and inexperienced students.