2 20 TEXTBOOK OF BOTANY 



237. Hairs and Similar Structures. — Hairs, which appear 

 upon many stems, are best studied in connection with 

 leaves, upon whose surfaces they grow in great number and 

 variety. Like root hairs, the hairs of stems and leaves, as 

 well as those that are often borne on sepals, petals, and other 

 flower parts, are outgrowths each of a single epidermal cell. 

 A hair sometimes remains single-celled ; sometimes it becomes 

 a row of cells ; sometimes it is branched. When present 

 in large numbers, hairs form a felt over the surfaces of stems 

 and leaves that helps to check the evaporation of water. 

 Many plants which, like the mullein, grow in extremely dry 

 soil, have very hairy stems and leaves. Winter buds are 

 often protected by hairs as well as by scales. Sometimes 

 the hairs form a woolly outer covering for the bud ; some- 

 times they are inside the protecting scales. The prickles of 

 roses, raspberries, blackberries, and gooseberries are like 

 hairs in that they grow from the surface of the stem ; but 

 some of the cells below the epidermis, as well as the epidermal 

 cells themselves, take part in their formation. These 

 prickles protect the plant against injury by animals, and 

 also, by penetrating objects with which they come in contact, 

 help the stems of some plants, such as climbing roses and 

 the hop, in climbing. Prickles look like thorns and serve 

 much the same purpose ; but that they are different in origin 

 is shown by the fact that they come off with the bark when 

 the latter is peeled off, while thorns (being either leaves or 

 branches) do not. 



238. Length of Life of» Stems. — The stems of many 

 plants live only through the growing season of a single year. 

 Such short-lived plants are called annuals. The cereal 

 grains are all annuals. So are cucumbers, peas, beans, and 

 many other garden plants. But some garden plants, like 

 the cabbage, carrot, and beet, and many wild plants, have 

 stems that live for two seasons. Such plants are biennials. 

 A biennial bears no flowers during the first year of its Hfe ; 



