46 BOTANY PART i 



or cell groups of a different structure and content to be found in an 

 otherwise uniform type of tissue. 



In the higher plants particular kinds of tissue may occur in 

 considerable amount and extend in unbroken connection for a distance 

 or through the whole plant body. These may often include several 

 associated kinds of tissue and constitute MORPHOLOGICAL TISSUE 

 SYSTEMS. Such compound associations of tissues may be characterised 

 structurally and have different main functions. The functions of the 

 different kinds of tissue within them tend to complement one 

 another. 



In a PHYSIOLOGICAL TISSUE SYSTEM are' grouped together all cells that agree iir 

 their main functions, irrespectively of their morphological connection, or of their 

 ontogenetic origin. Such physiological systems are thus something quite different 

 from morphological tissue systems. 



The tissue systems of the more highly organised plants can be 

 divided into two main groups: (1) the meristematic or formative 

 tissues ; (2) the mature or permanent tissues. 



A. The Formative Tissues 



These are also termed MERISTEMS and consist either of relatively 

 small cubical or isodiametric cells, or of prismatic, flattened, or elongated 

 cells with thin walls, abundant protoplasm, large nuclei, and few and 

 small vacuoles (cf. Fig. 2). The numerous cell divisions that occur in 

 their cells is characteristic. These formative tissues, from which the 

 permanent tissues are developed, are distinguished according to the 

 place and mode of their origin into PRIMARY and SECONDARY 



MERISTEMS. 



1. Primary Meristems. These arise by the division of the 

 germ cell and at first compose the whole embryo. Later they become 

 localised at the growing points of the branches and roots (Figs. 102, 

 157), where the increase in number of meristematic cells and the 

 formation of the rudiments of many lateral organs takes place 

 (apical growth). 



One or a number of the cells at the extreme tip of the growing 

 point always remain meristematic, and multiply by growth and 

 continued cell division following on this. The meristematic cells thus 

 produced, after undergoing further divisions, become gradually trans- 

 formed into cells of the permanent tissue. When there is a single 

 cell at the tip distinguished by its form and size from the other 

 meristematic cells it is called an APICAL CELL (Figs. 100, 101, 156); 

 when there are a number of cells in one or more layers they are 

 spoken of as INITIAL CELLS (Figs. 102, 157). The latter may resemble 

 apical cells, but are often more like the other meristematic cells. 



A short distance behind the growing point the similar cells of the 

 primary meristem begin to grow differently and give rise to strands 



