82 THE HUMAN SIDE OF TREES 



the cell also changes with age ; the envelope, delicate 

 and thin in youth, afterward receives accretions 

 and ornaments. Some cells have within a hollow 

 screw-way, like a winding stair; in others, the in- 

 side is covered with beautiful nettings, rings, flut- 

 ings, or lattices. Most cells thicken their casings, 

 as the oyster does, by adding new layers over the 

 older ones ; and, when their hollows are quite filled 

 up, they may rival stones and bones in hardness, 

 as, for example, the cells of the ironwood and the 

 ivory-nut. 



"As the cell-wall grows thicker, fluids and gases 

 penetrate its invisible pores with more difficulty; 

 and with continuous increase of thickness the liv- 

 ing protoplasmic bodies inhabiting its interior must 

 finally die for want of food. They in effect build 

 their own coffin, immure themselves living in their 

 own cell-prison. But a wonderful provision pre- 

 vents the food being entirely cut off. While the 

 cell-wall is arching itself up more closely and 

 thickly a few doors and windows are still left open 

 in it, through which communication may still take 

 place with the adjoining cells; this occurs by the 

 cell-wall not becoming strengthened at particular 

 points; and when, in the course of time, the shell 

 has become still thicker these places appear as pores 

 or canals which lead outwardlv from the interior of 



