STEM FORMS AND USES I45 



and air. In general, leaves seek the best possible light 

 exposure, and hence the normal growth of the stem is 

 upward, toward the Hght. There are exceptions, how- 

 ever, in the case of shade-loving plants that seek the 

 shelter of the forests, and certain winter green herbs like 

 the chickweeds, Indian strawberry, and dandelion, that 

 protect themselves against stress of weather by lying low 

 and hugging the earth. The same habit may temper both 

 the summer's heat and winter's cold, by shading the earth 

 around the roots and preventing too rapid evaporation in 

 the hot season, and by keeping them in contact with the 

 warm earth and preventing too rapid radiation in winter. 



206. The Surface of Stems, like that of leaves, may be 

 hairy, prickly, smooth, rough, etc., and the same terms are 

 used in describing them. The object of these adaptations 

 is the same as in leaves. Grooves and wings and hairs 

 may either be related to drainage and aid in the conduc- 

 tion of water, or they may help or hinder the visits of 

 certain insects and other animals. Some of these devices 

 are very ingenious, and have been imitated by man. The 

 sticky gum exuded from the upper nodes of the catchfly 

 {Silene) protects the flower against the visits of crawling 

 insects as effectively as would a strip of sticky fly paper ; 

 and our barbed-wire fences do not serve their purpose any 

 better than the prickles of the black- 

 berry and the cactus. In regard to 



207. Shape, stems are either round 

 (terete), flattened, square, triangular, etc. 

 Sometimes the shape is of great use in 

 helping to distinguish different kinds of 

 plants. In the mint family and its allies, 

 square stems are prevalent ; the sedges 

 are generally characterized by triangular 

 ones, and grasses by round, hollow, 

 jointed ctilms, or haulms, as they are 

 called, like those of wheat, oats, and 



rye. 288. — Culm of millet. 



ANDREWS'S BOX. — lO 



