THE STEM— ITS GENERAL STRUCTURE 



67 



Fig. 83. — C o L L E N - 

 chyMa in Wild 

 jewelweed or 

 Touch-me-not (Im- 



PATIENS). 



Fig. 84. —Grit Cells. 



times the walls of parenchyma cells in the cortex thicken 

 at the corners and form brace cells (Fig. 83) (collenchyma) 

 iox support ; sometimes the whole wall is thickened, form- 

 ing grit cells or stone cells (Fig. 



84; examples in 



tough parts of 



pear, or in stone 



of fruits). Some 



parts serve for 



secretions (milk, 



rosin, etc.) and 



are called latex 



tubes. 



The outer bark of old shoots consists of corky cells that 

 protect from mechanical injury, and that contain a fatty sub- 

 stance (suberin) impermeable to water and of service to 

 keep in moisture. There is sometimes a cork cambium (or 

 phellogen) in the bark that serves to extend the bark and 

 keep it from splitting, thus increasing its power to protect. 

 Transport of the "Sap." — We shall soon learn that the 

 common word " sap " does not represent a single or simple 

 substance. We may roughly distinguish two kinds of more 

 or less fluid contents : ( i ) the root zvater, sometimes called 

 mineral sap, that is taken in by the root, containing its 

 freight of such inorganic substances as potassium, calcium, 

 iron, and the rest; this root water rises, we have found, in 

 the wood -vessels, — that is, in the young or " sapwood " (p. 

 96); (2) the elaborated or organised materials passing back 

 and forth, especially from the leaves, to build up tissues 

 in all parts of the plant, some of it going down to the roots 

 and root-hairs ; this organic material is transported, as we 

 have learned, in the sicve-ttibcs of the inner bast, — that is, 

 in the "inner bark." Removing the bark from a trunk in 



