Herbs, Shrubs, and Trees 



97 



fact that in annuals, biennials, and a few perennials there is 

 no well-marked period of senility or old age. They die sud- 

 denly at maturity, immediately after 

 their period of greatest vigor. Trees 

 and shrubs, on the contrary, have a 

 distinct period of old age in which 

 the physiological processes are slowed 

 down gradually until the plants suc- 

 cumb to diseases and unfavorable 

 conditions which they could have 

 withstood in youth. 



Perennials classified according to 

 the persistent parts. All perennials 

 add new leaves, new stems, and new 

 roots each year; but they may be 

 classified roughly according to the 

 parts that persist from one season to 

 the next. 



Evergreen trees and shrubs are 

 perennial in all parts of the plant FIG. 58. Century plant (Agave), 



-, T-^ . , JIT showing rosette of fleshy leaves 



body. Deciduous tees and shrubs and flowering stem It is a 

 are perennial in their stems and roots, perennial, but, like an annual 

 Many herbaceous perennials, like the r a bienni ^' . ll dies w en ll 



* flowers and fruits. 



cat- tails, swamp mallows, peonies, 



trilliums, and bananas, have annual above-ground stems but 

 perennial underground stems and roots. Dahlias and sweet 

 potatoes have perennial roots. Potatoes and the Jerusalem 

 artichoke (a kind of sunflower) have perennial thickened 

 underground stems (tubers). Tulips and hyacinths have 

 perennial underground stems (bulbs). These examples show 

 that perennial plants have many different ways of bridging 



