Buds and Stems. 73 



services. The tuber of the Irish potato, for instance, is a 

 portion of a stem having for its chief function the storage 

 of materials ; the stems of cacti are green, and perform 

 the food-producing functions ; the underground stems of 

 Sagittaria and goldenrod creep along under the surface of 

 the gi'ound, producing new erect shoots here and there, 

 and so serve the purpose of multiplication ; the tendrils of 

 the grape and Virginia creeper, which seem to be stems, 

 have a supporting function. Indeed, as we shall learn in 

 the chapter on modified parts, the members of the plant 

 body may be put to quite as various services as the 

 economy of the plant may require. 



58. Habits of Stems. — The typical foliage stem is erect 

 or nearly so, and stands by its own strength. Climbing 

 plants do not possess the strength nor form of stem to en- 

 able them to acquire the erect position without the aid of 

 some support. An examination of the structure of stems 

 of climbing plants reveals the fact that they are relatively 

 lacking in the strengthening elements, while the tubes for 

 conducting water occupy a correspondingly larger space. 

 This is what we should expect, for plants that have ac- 

 quired the habit of clinging to other objects for their sup- 

 port have less need of strengthening elements, while their 

 long and slender stems, which lift the foliage to a con- 

 siderable height above the ground, require that a relatively 

 large space be given over to highways for water transport. 

 At the opposite extreme of habit stand the plants with 

 prostrate stems, such as Euphorbia serpejis. ■ These two 

 types of stems, the climbing and the prostrate, between 

 which all degrees of gradation may be found, are adapted 

 to quite different habitats, — the climbing stems to shady 

 situations, such as dark woods, or the shady side of banks 

 or buildings, and the prostrate stems to situations along 



