146 General Botany 



down gradually until the plants succumb to diseases and unfa- 

 vorable 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 body. Deciduous trees and shrubs are perennial in their 

 stems and roots. Many herbaceous perennials, like the cat- 

 tails, grasses, mints, 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 and buds (bulbs). These 

 examples show that perennial plants have many different ways 

 of living over unfavorable seasons like periods of cold or drought. 



There seems to be no limit to the length of Hfe of some perennial 

 herbs, like ferns, the May apple, Solomon's seal, and certain 

 grasses and mints. The older parts die each year, and new parts 

 form at the growing ends of the underground stems. The plants 

 change their locations slightly each year, one end of the stem 

 growing forward and the other end dying away. There is no 

 apparent reason why such plants should not live indefinitely, 

 perhaps longer than the oldest tree ; but no one part of the plant 

 lives for a long time. 



Herbs, shrubs, and trees. Shrubs and trees have woody 

 stems. The stems of herbs have comparatively little woody 

 tissue. Our garden and field crops are all herbaceous plants. 

 Their stems contain little woody tissue, and in temperate climates 

 the above-ground parts live only during a single growing season. 



The principal difference between shrubs and trees lies in the 

 fact that shrubs develop numerous slender above-ground stems 

 from a single base, while trees develop a single stem or trunk. 



