ROOTS AND THEIR USES 



171 



bark ; but the bark of roots does not grow so fast nor be- 

 come so thick as that of stems often does. 



193. Occurrence of Adventive Roots. — Not a few plants, 

 like the Indian com, produce adventive roots regularly and 

 in considerable numbers. A 

 potato shoot growing from 

 an eye of a tuber sends out 

 adventive roots from its 

 lower part. Plants with 

 long underground stems, hke 

 the may-apple, as well as 

 many trailing plants, pro- 

 duce adventive roots from 

 time to time. As the older 

 parts of such a plant, in- 

 cluding its primary root, 

 die, its root system comes 

 to consist entirely of adven- 

 tive roots. Some climbing 

 plants form adventive roots 

 in large numbers ; but such 

 plants usually remain con- 

 nected with the soil by 

 means of a primary root, 

 and the adventive roots are 

 useful chiefly to assist the 

 plants in climbing. Many 

 plants form adventive roots 

 in response to the stimulus of a wound. In some kinds 

 of trees, for example, girdhng (see § 232) is followed at 

 first by the formation of adventive roots on the upper 

 edge of the wound, and in other kinds it is followed by the 

 formation of adventive buds (which may develop into 

 branches) on the lower edge of the wound. If a leaf of a 

 rex begonia is cut into several pieces and these are stuck 



Fig. 106. — ,A leaf of Bryophyllitm 

 which is producing new plants — a 

 case of vegetative multiplication. 

 This leaf was placed on moist sand, 

 and in a few days adventive buds and 

 adventive roots developed wherever 

 the veins extended to the edges of the 

 leaf. 



