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 
