CELLS AND TISSlJi:s 



II.-) 



but in general they are alive and act as storage cells. Some of the 

 parenchyma cells, called collectively collnichynw, become f liickcncd 

 at the corners, as seen in a cross .section, and serve as strengthcnii.n 



'menStQWatiC P'^'^^'^.X"'-^ colknch^roa sderenchv^xt 

 tissues "tancCameatial tissues 



oooo 



■ Sedition 



epjdermis 



"plant- -, I", y, ... 



... i:icxir^ :jcylem M '.phloem 



prouective t-issues <:tondxxctin<^ tissues 



Types of plant tissue cells. 



units in the outer part of the stem. The walls of the other funda- 

 mental tissue cells become much thickened and are called sclcrcrichyma 

 cells, which may become fibrous, helping to .support the stem, while 

 others form stone cells making up the covering of nuts antl other 

 hardened parts. 



The Conducting Tissues. In the liigher plants, woody bundles 

 of elongated cells act as tubes for the conduction of water and food 

 substances. The water-conducting tissues are collectivelj' known 

 as the xylem and consist largely of supporting dead cells (trachcids) 

 impregnated with a strengthening substance called lignin, and long 

 tubular cells (vessels) which have lost their cross walls. Scattered 

 amongst them are various other types of cells, including jiarenchynia. 

 The tissues which conduct food materials down the stem from the 

 leaves, where food is made, are known collectively as the phlonn. 

 The characteristic conducting cells of the j^hloem are known as sifir 

 tubes, which have perforations in the end or .side walls known as the 

 sieve plates. Long threads of cytoplasm pa.ss through these holes. 



