CHAP, iv.] Origin of Tissues. 319

in scientific monographs ; the text-books also could now be supplied with an abundance of figures, and this greatly pro- moted the general understanding of things which could other- wise be seen only under the glass of each observer. From the close of the 16th century wood-cuts had fallen more and more into disuse, and had ' been replaced by copper-plates ; after 1840 wood-engraving was restored to its old rights and was found to be a more convenient method of pictorial illustration, especially for text-books ; thus Schleiden's ' Grund- ziige' of 1842, von Mohl's ' Vegetabilische Zelle' of 1851, Unger's and Schacht's text-books were enriched with many and sometimes very beautiful w r ood-cuts. Lithographs were generally- preferred for periodicals and monographs ; the ' Botanische Zeitung,' founded by Mohl and Schlechtendal in 1843, and till after 1860 the chief organ for shorter phytotomic com- munications, was illustrated by a large number of beautiful prints from the establishment of the Berlin lithographer, Schmidt.

i. DEVELOPMENT OF THE THEORY OF CELL-FORMATION

FROM 1838 TO 1851.

Since we are here dealing with questions of fundamental importance not only to one branch of botanical study but to the whole science of botany, and even to the rest of the natural sciences, it seems imperative that we should follow step by step the founding and perfecting of the theory of the cell, as far as is possible in the limited space at our com- mand ; we shall deal with the sexual theory further on in a similar manner.

As usually happens in the inductive sciences, the period of strict inductive investigation into cell-formation was preceded by a still longer time, during which botanists ventured to put forward general theories in reliance on highly imperfect obser- vations. We have already seen how Caspar Friedrich Wolff

in 1 759 made cells originate as vacuoles in a homogeneous