258 EXPERIMENTS WITH PLANTS 



grain of the wood, which runs from the center out- 

 ward to the bark at right angles to the ordinary grain. 

 On splitting an Oak stem squarely in two, this silver 

 grain is very conspicuous and, on testing, is found 

 to be filled with starch. In sections (Figs. 135 and 

 138, mr) the silver grain is seen to consist of long, 

 tubular cells like the wood- cells (ducts are absent) 

 running in a radial direction, at right angles to the 

 course of the wood -cells. The silver grain is called 

 by botanists the medullary rays (medulla means pith) ; 

 they serve as channels of communication between the 

 wood- cells and the cells of the outer portion of the 

 stem ; they may convey food, water or gases. 



Why is so much starch conveyed to the wood ? Little 

 or no growth is taking place in that portion of the 

 wood to which most of the starch is conveyed ; 

 moreover, the amount of starch increases instead of 

 diminishes during the growing season. Just after the 

 leaves have fallen off the wood is very rich in starch, 

 while the fallen leaves contain practically no food 

 substances of any sort (test this matter). It would 

 appear that the starch is conveyed from the leaves to 

 the wood for the purpose of storing it up there. Not 

 only in the wood but also in rind or cortex do we find 

 starch stored up at this time of year. Later on, during 

 the winter, we find that some of the starch has 

 disappeared, but on testing we find an increase in the 

 amount of sugar; we conclude, therefore, that a part 



