TISSUES. 47 



assimilated food-material contained in the cells. Sieve- 

 tubes are present in the bast or phloem of most flowering 

 plants, and also in the stem of some of the large brown alga. 



Differentiation of Tissues. — In many af the simpler 

 multicellular plants the constituent cells are, during the 

 vegetative phase, similar in structure and function, but in 

 the higher Cryptogams and Phanerogams groups of similar 

 cells occur, which differ in structure and function from other 

 groups of cells that surround them, thus constituting a dis- 

 tinct form of tissue. Two such forms of tissue of primary 

 importance may be distinguished. 



Parenchyma : The cells are more or less isodiametric, or 

 not much longer than broad, and are in contact by broad 

 surfaces; intercellular spaces are usually present; pith, 

 succulent parts of fruits, the entire tissues of many sea- 

 weeds, etc., are examples. 



Prosenchyma : The cells are very much longer than 

 broad, the ends are tapering and overlap, and there are no 

 intercellular spaces; bast and wood fibres are examples 

 of this class. 



When the cells of either of the above systems become 

 much thickened, hard, and often dark coloured, the tissue 

 is termed sclerenchyma ; examples, plum-stone, shell of 

 cocoa-nut, most kinds of wood. 



Meristem, or generating tissue, is the name given to 

 groups of cells that still possess the power of dividing, and 

 is consequently always present at those points where growth 

 due to the increase in number of cells is taking place, as 

 the tips of roots, stems, etc., which are spoken of as 

 growing-points. Meristem, a primitive form of parenchyma 

 is homogeneous in structure, and is the tissue from which 

 all other tissue-systems are subsequently differentiated. 



