236 



HOW CKOP3 GROW. 



cap to 

 root in 



protect 



its act 



the true termination or living point of the 

 of penetrating the soil. Fig. 36 represents 



a magnified section of part of a 



barley root, showing the loose 



cells which slough off from the tip. 



These cells are filled with air in- 



stead of sap. 

 A most strik- 



ing illustra- 



tion of the 



root - cap is 



furnished by 



the air-roots 



of the so- 



called Screw 



nus odoratis- 



simus,) exhibited in natural dimen- 

 sions, in fig. 37. These air-roots issue 

 from the stem above the ground, and, 

 growing downwards, enter the soil, 

 and become roots in the ordinary sense. 



When fresh, the diameter of the 

 root is quite uniform, but the parts 

 above the root-cap shrink on drying, 

 while the root-cap itself retains nearly 

 its original dimensions, and thus 

 reveals its different structure. 



Distinction between Root and 

 Stem. Not all the subterranean parts 

 of the plant are roots in a proper Fig. 37. 



sense, although commonly spoken of as such. The tubers 

 of the potato and artichoke, and the fleshy horizontal parts 

 of the sweet-flag and pepper-root, are merely underground 

 stems, of which many varieties exist. 



These and all other stems arc ea,sily distinguished from 



