THE STEM— ITS GENERAL STRUCTURE 67 



times the walls of parenchyma cells in the cortex thicken 

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

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

 ing grit cells or stone cells (Fig. 



84; examples in j^P^^fe^^^/I 



tough parts of 



pear, or in stone 



of fruits). Some 



parts serve for 

 FIG. 83. - CoLLEN- secretions (milk, 

 CHYMA IN Wild rosin, etc.) and 



JEWELWEED OR W A J f 



Touch-me-not (IM- ^'^^ called latex 



PATIENS). tubes. FIG. 84. -GRIT CELLS. 



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 

 ph'ellogen) 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 water, 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) \!aQ, elabo7'ated ox organized materials ^2i%zm^\)2,c\i 

 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 sieve-tubes of the inner bast, — that is, 

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



