CHAP, iv.] Classification of Tissues. 345

those states of development, in which they are not yet adapted to subsequent physiological functions. The combination of morphological and physiological points of view in the determin- ation of facts has maintained itself longer in this part of botanical study than in any other ; but here too ideas and opinions were gradually sifted and cleared up under the influ- ence of the modern study of the history of development, though it was not till after 1850 that the determination of the chief points in the theory of cell-formation left the leading phytotomists at liberty to devote themselves to histological questions.

How little advance had been made towards the true under- standing of the varieties of forms of tissue in the higher plants before 1850 is shown, for instance, by Schleiden's account of tissues on page 232 of his 'Grundziige' of 1845, where parenchyma, intercellular substance, vessels, vascular bundles, bast-tissue, bast-cells in Apocyneae and Asclepiadeae, latici- ferous vessels, felted tissue, epidermal tissue, are discussed in this succession in co-ordinated sections of the text. It is obvious that no well-ordered view of the whole cellular structure of a plant of the higher order could be obtained in this way. Further on in the same work, where Schleiden attempts a classification of vascular bundles, which he dis- tinguishes into closed and open, and assigns the latter to Dicotyledons, we find the cambium-layer named as the outer boundary of these open vascular bundles ; the bast which lies outside the cambium was therefore not considered to be a part of the open vascular bundles, and this necessarily excluded any profitable comparison of the circumstances in Monocoty- ledons and Dicotyledons. The case is still worse in many respects in Schacht's work already mentioned, 'Die Pflanzenzelle' of 1852, where under the heading 'Kinds of vegetable cells' the histology is discussed in the following co-ordinated sections ; the swarm-filaments of Cryptogams, the spores of the same,

pollen-grains, cells and tissue of Fungi and Lichens, cells and