236 



HOW CROP,} GltO .V. 



cap to protect the true termination or living point of the 



root in its act of penetrating the soil. Fig. 06 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 

 Piue, (Pai teta 

 nus odorutis- 



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 



sense, although commonly spoken of as such. The tubers 



of the potato and artichoke, and the fleshy horizontal parts 



of the sucd-llaLT and pepper-root, are merely underground 



stems, of which many varieties exist. 



These and all other stems are easily distinguished from 



Fiir. 37. 



