CHAPTER II. 



MERISTEMATIC TISSUES. 



/. GENERAL CONSIDERATIONS. 



The gradual increase of histological differentiation, which is so striking 

 a feature of the evolutionary history of the vegetable kingdom, is 

 equally characteristic of the ontogenetic development of the shoot or 

 root of any highly organised plant. A transverse or longitudinal 

 section taken from the distal end of such an organ reveals the fact, 

 that this region displays no trace of the complex structure which charac- 

 terises the older portions of the organ, but is entirely made up of thin- 

 walled cells with abundant protoplasmic contents. There is of course 

 no sharp boundary dividing the homogeneous cell-mass of the growing- 

 point ?>1 from the fully differentiated tissue-systems of the adult organ. 

 ( 'lose behind the actual growing-point the uniform cell-mass becomes 

 differentiated into several distinct layers, the cells of which, however, 

 still retain certain common features, such as abundance of protoplasm, 

 thinness of cell-walls and the power of active division. As the dis- 

 tance from the growing-point increases, the distinctive characteristics of 

 the several tissues and tissue-systems come more and more into promi- 

 nence, and the organ thus gradually attains to a degree of differentiation 

 commensurate with the extent to which it embodies the principle of 

 division of labour. 



The above-described tissues, located in the growing-points and in 

 the immediately adjacent distal regions of the shoot and root, represent 

 the embrvonic stages of the bulk of the tissue-svstems which are 

 discussed in the subsequent chapters of this work : the meristematic 

 tissues may, in other words, be termed formative tissues in contrast to 

 the executive permanent tissues. The ontogenetic development of 

 every permanent tissue may, in fact, be divided into three stages. In 

 the first of these the tissue becomes marked off from the general 

 embryonic tissue of the growing-point, and undergoes a certain amount 



