4 INTRODUCTION. 



merislem \ In structure the cells of meristem are characterised by having a delicate 

 homogeneous membrane (only in certain exceptional cases thickened and with 

 flattened pits), and homogeneous, finely granular protoplasm with a nucleus, but 

 ^vilh no further recognisable structural elements. By reason of their constant 

 division, they are throughout in uninterrupted connection with one another. 



In each system of the meristem the divisions pass through a definite number of 

 stages, till they gradually cease. According as this happens, the cells, first formed as 

 members of the meristem, assume those properties, by which the further sorts of tissue 

 are distinguished : great increase in volume takes place, and changes of structure and 

 form ; while the latter may result in a partial loosening of the original uninterrupted 

 connection, i. e. in the appearance of intercellular spaces. 



As compared with the meristem, the tissue elements derived from it attain a 

 great constancy both of form and structure. They have accordingly been termed 

 permanent tissue, fixed tissue, or mature tissue. If the idea of tissue be understood in 

 the general sense stated above, the meristem also is naturally included : we distinguish 

 then on the one hand meristem, or formative tissue, on the other permanent tissue, as 

 the two main categories of tissue. Merely for shortness of expression, however, the 

 term tissue will be used also for permanent tissue in opposition to meristem. In this 

 sense, and according to the previous explanation, the following pages will deal with 

 the vegetative tissues, which serve as vegetative organs. 



Comparative investigation shows that the cells of the meristem universally cor- 

 respond very closely in the general character of their structure, and the same may be 

 said of the main phenomena of the vegetaUve process in the plants we are engaged 

 with. Answering to this correspondence of origin and functional adaptation, the 

 forms of tissue in the whole group on which we are engaged correspond in their 

 main properdes, notwithstanding frequent modifications to suit special cases, and the 

 same few tissue-forms everywhere occur. 



Tissue-elements of every sort are derived from the cells of the meristem, each 

 has originally the properties of a cell. Accompanying definite development there first 

 appears the fundamental difference, that certain elements retaiii during their life the 

 structure and all the characteristic properties of typical cells, others lose the cell-nature. 

 The former are composed of a completely closed cell-membrane and active proto- 

 plasm, with a nucleus and cell-contents ; they retain the power of independent growth, 

 and remain capable of division ; in consequence of this property a meristem may 

 ^gain arise from them, and this, in antithesis to the original, is distinguished as 

 seco7idary meristem (Nageli, 1. c). The latter lose with their development the power of 

 division, and of independent growth; usually they cease entirely to grow, but in many 

 cases a real lasdng growth of such elements occurs, resulting from their nourishment 

 by adjoining cells. In their structure, the loss of the cell-nature is indicated either 

 by the complete disappearance of the protoplasmic body, its place being filled by 

 other bodies, usually air or fluids ; or by its suffering characteristic changes, which 

 vary according to the individual cases. The latter observation is made with special 



' Niigeli, Beitrage, I. p. 2. — Schleiden (Grundz. 3 Aufl. I. p. 253) and Karsten (Veg. Org. d. 

 Palmen) include these masses under the wide term Camblmn; Unger (Anat. und Physiol, p. 180) 

 calls them formative cells; Schacht (Pflanzenzelle, p. 165) primary parenchyma. 



