Green Leaves at Work 103 



But even when man has interfered with this pro- 

 gramme the starch-grains are not without their 

 use. They close the pores of the wood, mak- 

 ing it almost impenetrable, and hence peculiarly 

 adapted to certain economic uses. "Winter- 

 hewn timber is almost exclusively employed for 

 staves," says the Scientific American. "With 

 staves made from summer-wood the contents of 

 the barrel are subject to evaporation through the 

 pores." 



The stored-up starch-grains in tubers and seeds 

 have very characteristic forms. 



Those which we find in the tubers of the Indian- 

 shot look like clam-shells, and those of the 

 potato are uneven ovals. Those which we find 

 in grains of corn are very small and angular, 

 like particles of sand, and those of barley, 

 wheat, and rye are lens-shaped (Fig. 19). When 

 these starchy roots and seeds begin to grow the 

 starch will be changed into fluid glucose and then 

 drawn from cell to cell till it reaches the push- 

 ing tips of stems and roots. 



The water which ascends from the roots of a 

 growing plant into its leaves holds in solution 

 about as much mineral matter as is contained in 

 ordinary well-water. The warmth of the summer 



