PLANT ORGANS AND ORGANISMS 
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qf starch, etc. is gradually relegated to the outer wood layers, since 
the inner layers, step by step, lose their protoplasmic contents and 
Fig. 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, f 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- 
