156 



PLANT BIOLOGY 



mature, is of large size, with a single woody trunk and branches. 

 This trunk may extend up nearly to the top of the tree, as in 

 the case of the pines and spruces, or some distance above the 

 ground the trunk may divide into branches, as is true in the 

 elms and maples. 



A shrub, on the other hand, is usually of smaller size even 

 when fully grown than is a tree ; it commonly does not have a 

 single trunk, but several woody stems which often start from 

 the ground level, as in the lilac, rose, 

 and witch hazel. Both shrubs and 

 trees are alike in that their stems and 

 branches do not die down to the ground 

 at the end of the season. 



An herb, as the term is used in plant 

 biology, is a plant of relatively small 

 size, with comparatively little woody 

 material in its stem, which dies down to 

 the ground level at the close of the 

 season. Such are beans, corn, and 

 morning glories. The roots or under- 

 ground stems of some herbs for ex- 

 ample, dahlias, carrots, and parsnips 



remain alive ready for growth the next year. These facts 

 suggest another method of classifying plants, namely, as : 



FIG. 80. An herb. 



166. Annuals, biennials, and perennials. When a plant 

 attains its maturity in one season's growth and then dies, as 

 do beans, corn, and morning glories, such a plant is called an 

 annual. Many plants which have fleshy roots, like the beet, 

 carrot, and parsnip, do not produce flowers and seeds until the 

 second year. During the first season after the seed is planted 

 the food manufactured in the leaves passes down the plant 

 and is stored beneath the ground. At the end of the season 



