THE STRUCTURE OF WOOD. 



15 



were first called "cells" by Eobert Hooke, in 1667, because of their 

 resemblance to the chambers of a honeycomb. The 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 protoplasm, con- 

 stitutes the primal stuff 

 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 

 protoplasm, and as yet, 

 unaltered. They form 

 embryonic tissue, i. e. 

 one which will change. 

 One change to which any 

 cell filled with proto- 

 plasm is liable is divi- 

 sion into two, a new par- 

 tition wall forming with- 

 in it. This is the way 

 plant cells increase. 



In young plant cells, 

 the whole cavity of the 

 chamber is filled with 

 protoplasm, but as the 

 cells grow older and 

 larger, the protoplasm 

 develops into different 

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

 ties within the 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 

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



Fig. 5. Young Stem, Magnified 18^ Diameters, Show- 

 ing Primary and Secondary Bundles. By Courtesy of 

 Mrs. Katharine Golden Bitting. 



E, epidermis, tbe single outside laver of cells. 

 C, cortex, the region outside of the bundles. 



HB, hard bast, the black, irregular ring protecting 

 the soft bast. 



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

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

 wood. 



W, wood, segments showing pores. 



MR, medullarj- rays, lines between the bundles con- 

 necting the pith and the cortex. 



MS, medullary sheath, the dark, irregular ring just 

 inside the bundles. 



P, pith, the central mass of cells. 



