GROWTH 



147 



of brick (the wood) surmounted by a coping of tile (the bast); 

 and by assuming that just at the junction between brick and tile 

 a bricklayer (the cam])ium) is able repeatedly to insert a brick 

 and a tile, the wall thus mounting upward and carrying an 

 ever-thickening coping of tile on its top. 



I Old Bas-i- 



Nlew Basi 



Camb'ium 



i- Klew IVoool 



Old Wood 



Fig. 73. — Cambium of pine stem actively producing new wood and bast. 

 The cambium is densely filled with protoplasm. The youngest cells of wood and 

 bast, lying next the cambium, are still small, but as they grow older they soon 

 expand to their mature size. The new wood-cells are still very thin-walled and 

 retain a lining of cytoplasm, which later disappears entirely. 



As a consequence of this method of growth, the youngest layers 

 of the wood are the outermost and the youngest laj-ers of the bast 

 are the innermost; and the past history of these two tissues, as it 

 is preserved in their structure, should thus be read in opposite 

 directions (Fig. 75). In woody plants, where growth in thickness 

 is considerable and continuous, the bast, because of its rather 

 delicate texture and the strain which is put upon it, becomes much 

 stretched and crushed in its outer layers. These are ultimately 



