256 TEXTBOOK OF BOTANY 



branching of the younger twigs of ash, maple, and buckeye; the in- 

 numerable and slender ultimate branches of the elm; the papery bark 

 and elongated lenticels of some birches and cherries; the resinous buds 

 of horse-chestnut; the star-shaped pith visible in cross sections of oak 

 twigs; and the chambered pith of walnut and hackberry are a few ex- 

 amples of distinguishing characters that may be selected. Such characters 

 may be used as a basis for making a key for the ready identification of 

 woody plants. 



Herbaceous aerial stems. Stems of herbs are usually readily distin- 

 guished from those of trees and shrubs by the comparatively smaller 

 amounts of woody tissues. Some annual stems, such as those of sunflower, 

 giant ragweed, and a few asters, become woody at maturity in certain 

 habitats, but they are classed with herbaceous plants. It is really a mat- 

 ter of opinion whether certain semi- woody plants should be called herbs. 

 Manv herbs are annuals or biennials, in contrast to trees and shrubs 

 which are generally perennials. Many herbaceous plants have perennial 

 underground stems some of which are very hard and woody; but the 

 annual aerial shoots that develop from these stems are herbaceous. 

 Herbaceous stems are frequently green and have stomates. 



Although many external features common to woody stems are absent, 

 herbaceous plants are nevertheless often identified by certain features of 

 their stems, such as shape, color, amount of hairy covering, and the 

 presence of certain types of prickles, thorns, and tendrils. 



Rhizomes and runners. A common type of underground stem is repre- 

 sented by the rhizome of bluegrass, Johnson grass, cord grass, Canada 

 thistle, and Solomon's seal. Rhizomes grow horizontally at some depth 

 below the surface of the soil, have scale-like, non-green leaves, and 

 axillary lateral buds from which aerial branches develop at certain 

 seasons of the year. Roots generally appear on the ventral sides of a 

 rhizome at the nodes. The rhizome may be thick and fleshy ( Solomon's 

 seal. Fig. 91 ) , or slender and woody ( many grasses ) . 



Slender, prostrate stems, such as occur in strawberry and some ferns, 

 are often termed runners or stolons. These prostrate stems and all under- 

 ground stems are excellent means of vegetative propagation, both natu- 

 rally and artificially. They are distinguished from roots by the presence 

 of nodes and internodes. The "turf" of lawns and meadows is a shallow 

 layer of soil held together by an interwoven mass of rhizomes and the 

 accompanying roots. 



