THE STEM— ITS GENERAL STRUCTURE 



6$ 



larger, even if the tree should Hve a century. It is not 

 quite inclosed, however, for the narrow layers of soft cells 

 separating the bundles remain be- 

 tween them (Fig. y8), forming ra- 

 diating lines called medullary rays 

 or pith rays. 



The Several Plant Cells and their 

 Functions* — In the wood there are 

 some parenchyma cells that have 

 thin walls still, but have lost 

 the power of di- 

 vision. They are 

 now storage cells. 

 There are also 



^ 





ts;" 



an 



^ 

 ^==^ 



Fig. 78. — Arrangement OP 

 Tissues in Two - year - 

 OLD Stem of Moonseed. 



wood fibres which /, pith; /, parenchyma. The fibro- 



are thick-walled -scular bundles, or wood 



strands, are very prominent, with 

 Fig. 79. — Markings and rigid (h, Fig. thin medullary rays between. 



IN Cell Walls rr^x , , . .. ^ 



OF Wood Fibre '^j, auQ servo TO suppovt the sap-canals 



J/, spiral ;««, annular ; Or woocl vcsscIs {ov trachcids) that are 

 .jf , scaianform. formed by the absorption of the end 



walls of upright rows of cells ; the canals 

 pass from the roots to the twigs and even 

 to ribs of the leaves and serve to transport 

 the root water. They are recognized (Fig. 

 79) by the peculiar thickening of the wall 

 on the inner surface of the tubes, occur- 

 ring in the form of spirals. Sometimes the 

 whole wall is thickened except in spots 

 called ///J- (^, Fig. y6). These thin spots 

 (Fig. 80) allow the sap to pass to other 

 cells or to neighbouring vessels. 



The cambium, as we have seen, consists 

 of cells whose function is gyowtlL. These 



Fig. 80. — Pits in 

 THE Cell Wall. 



Longitudinal section of 

 wall at h^ showing 

 pu borders at o, o. 



