PLANT ORGANS AND ORGANISMS 151 



of starch, etc. is gradually relegated to the outer wood layers, since 

 the inner layers, step by step, lose their protoplasmic contents and 



PIG. 77. Diagrammatic representation of a block of pine wood highly mag- 

 nified, a, Early growth; b, late growth; c, intercellular space; d, bordered pit in 

 tangential wall of late growth; m, /-and e, bordered pit in radial wall of early 

 growth from different points ot view; h, row ot medullary cells for carrying food; 

 g, row of medullary ray cells for carrying water; k, thin place in radial wall of ray 

 cells that carry food (From Stevens.) 



power of conducting sap and become filled with extractive, resinous 

 and coloring matters. The outer whitish layers of wood which con- 



