THE SHOOT. 95 



to die at the close of the growing season, an underground 

 portion really survives, and sends up the new shoots. Such 

 plants, if they live for two years, are called biennials ; or, 

 if they live for several or many years, are called perennials. 



Fig. 108. — Shoots of Vella spinosa, showing thorns. Natural size. — After Kerner. 



The shoot may be composed mainly of soft tissues, and 

 persist underground, where it is protected against unfavorable 

 conditions, such as drought and cold, and especially against 

 sudden changes ; or it may be composed mainly of mechan- 

 ical tissues, and be fully exposed, as are the shoots of trees. 

 In these cases the leaves generally perish and drop off an- 

 nually, but in the "evergreen" plants they live more than 

 one growing season. 



