TI-IE STRUCTUEIi OF WOOD. 



15 



were first called "cells" by Eobert Hooke, in 1G67, because of their 

 resemblance to the chambers of a honeycomb. Tlie walls of these 

 "cells" are their most prominent feature and, when first studied, 

 were supposed to be the essential part ; but later the slimy, colorless 

 substance which filled 

 the cells was found to 

 be the essential part. 

 This slimy substance, 

 called protojDlasm, con- 

 stitutes the primal stulf 

 of all living things. The 

 cell walls themselves are 

 formed from it. These 

 young cells, at the apex 

 of a stem, are all alike, 

 very small, filled with 

 protojjlasm, and as yet, 

 unaltered. They form 

 embryonic tissue, i. e. 

 one whicli will change. 

 One changi? to which any 

 cell filled with proto- 

 plasm is liable is divi- 

 sion into two, a new par- 

 tition wall forming witli- 

 in it. This is the way 

 plant cells increase. 



In young plant cells, 

 the whole cavity of tlie 

 chamber is filled witli 

 protoplasm, but as the 

 cells grow older and 

 larger, the protoplasm 

 develops into difl'ereut 

 parts, one part forming tlie cell wall and in many cases leaving cavi- 

 ties within tlie cell, which become filled with sap. The substance of 

 the cell wall is called cellulose (cotton and flax fibers consist of al- 

 most pure cellulose). At first it has no definite structure, but as 

 giTjwtli goes on, it may become thickened in layers, or gummy, or 



Fiy. ^. Y<>un;j- Sleiii, ^layiiified 18^2 Diameters, Sliow- 

 iril,'Primar.v and Secondary Bundles. /^^' Conrtcsy of 

 Mrs. Katharine Gotde7i Bitting. 



E, epidermis, the single outside laver of cells. 



C, corte-\', the re^non outside of tin- bundles. 



HB, hard bast, the blacl,, irreyular rin^^ proiecting- 

 the soft bast. 



SB, soft bast, the liylit, crescent-shaped parts. 

 Ca, cambium, the line between the soft bast and the 

 wood. 



W, wood, seg-nients showing- pores. 



MR, medullary rays, lines between tire bundles con- 

 necting the pith and the cortex. 



MS, medullary sheath, the dark, irregular ring just 

 inside tlie bundles. 



P, pith, the central mass of cells. 



