368 



HOW CROPS GROW. 



When fresh, the diameter. 'of the root is quite uni- 

 form, but the parts above the root-cap shrink on dry- 

 ing, 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 pobato 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 are 

 easily distinguished from true roots 

 by the imbricated buds, of which 

 ■ indications may usually be found on 

 their surfaces, e. g., the eyes of the 

 potato-tuber. The side or second- 

 ary roots are indeed marked in their 

 earliest stages by a protuberance on 

 the primary root, but these have n oth - 

 ing in common with the structure of 

 true buds. The onion-bulb is itself 

 a fleshy bud, as will be noticed subse- 

 quently. The true roots of the onion are the fibers 

 which issue from the base of the bulb. The roots of 

 many plants exhibit no buds upon their surface, and are 

 incapable of developing them under any conditions. 

 Eoots of other plants, such as the plum, apple, and pop- 

 lar, may produce buds when cut off from the parent 

 plant during the growing season. The roots of the 

 former perish if deprived of connection with the stem 

 and leaves. The latter may strike out new stems and 



Fig. 37. 



