Chap, iv.] Origin of Tissues. 319 



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 more into disuse, and had been replaced by copper-plates ; 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 wood-cuts. Lithographs were generally 

 preferred for periodicals and monographs ; the ' Botanische 

 Zeitung,' founded by Mohl and Schlechtendal in 1843, and 

 till after i860 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. 



1. Development of the Theory of Cell-formation 



FROM 1838 TO 1 85 1. 



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 Wolflf 

 in 1759 made cells originate as vacuoles in a homogeneous 

 jelly, and how this view was adopted in all essential points by 



