76 OUTLINES OF PLANT LIFE. 



action of any extraordinary cause, but more commonly injury 

 of one sort or another acts as a stimulus to the production of 

 such buds. Buds which do not originate in regular succes- 

 sion on the parent shoot (i.e., the younger nearer the apex) 

 are called adventitious buds. 



Adventitious buds may arise upon stems, leaves, or roots. 

 They are most commonly and abundantly produced upon 

 stems and roots. 



92. Dormant buds. — Many buds continue to grow without 

 interruption from the time of their formation, but more cease 

 to develop after they have reached a certain stage. Such 

 buds may remain dormant for a considerable period, and 

 may even be overgrown and completely enclosed by the 

 wood upon old shoots. The bud in this case grows slowly 

 and maintains itself near the surface of the wood. It is quite 

 possible that these dormant buds should for some reason 

 begin to develop later, when they are liable to be confounded 

 with adventitious buds. 



In case thej have been buried by the growth of tissues over them, the 

 shoots which they produce will seem to come from the interior of the 

 organ upon which they are borne. This apparent internal origin must 

 not be confounded with the real internal origin of roots. 



Since in most cases lateral buds have a definite relation to 

 the leaves, the shoots which arise from them will have a 

 similar relation. But, as many buds are produced which 

 never develop into branches, this relation is often obscure 

 and difficult to see. 



93, Special forms. — ^The primary shoot may grow under- 

 ground, in which case its stem usually takes a horizontal 

 direction and becomes much thickened for storage of reserve 

 food (^ 196), while its leaves are so reduced as to be scarcely 

 recognizable. Such a shoot is a rhizome. When the primary 

 stem is short, erect, and crowded with thickened leaf bases it 

 forms a bulb, as in the hyacinth and onion. When the 



