16 



WOOD AND FOREST. 



hardened into lignin (wood), according to the function to be per- 

 formed. Where there are a group of similar cells performing the 

 same functions, the group is called a tissue or, if large enough, a 

 tissue system. 



When cells are changed into new forms, or "differentiated," as it 

 is called, they become permanent tissues. These permanent tissues 

 of the tree trunk constitute the various parts which we have noticed, 

 viz., the rind, the pith and the wood. 



The essentially living part of the tree, it should be remembered, 

 is the protoplasm : where there is protoplasm, there is life and 



Fig-. 6. Three Stag-es in the Development of an Exogenous 



Stem. P, pith; PB, primary bast; SB, secondary bast; C, 



cambium; PR, pith ray; PW, primary wood; SW, secondary 



wood; PS, procambium strands. After Boulger. 



growth. In the stems of the conifers and broad-leaved trees some- 

 times together called exogens this protoplasm is to be found in the 

 buds and in the cambium sheath, and these are the growing parts of 

 the tree. If we followed up the sheath of cambium which envelopes 

 a stem, into a terminal bud, we should find that it passed without 

 break into the protoplasm of the bud. 



In the cross-section of a young shoot, we might see around the. 

 central pith or medulla, a ring of wedge-shaped patches. These afe 

 really bundles of cells running longitudinally from the rudiments of 

 leaves thru the stem to the roots. They are made of protoplasm >4ind 

 are called the "procambium strands," Fig. 6. 



