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 fruits are agreeable as human food, and some of them have been greatly enlarged or "improved" by the arts of the cultivator. The seeds are usually indigestible.

Burs.—Many seeds and fruits bear spines, hooks, and hairs, which adhere to the coats of animals and to clothing. The burdock has an involucre with hooked scales, containing the fruits inside. The clotbur is also an involucre. Both are compositous plants, allied to thistles, but the whole head, rather than the separate fruits, is transported. In some compositous fruits the pappus takes the form of hooks and spines, as in the "Spanish bayonets" and "pitchforks." Fruits of various kinds are known as "stick tights," as of the agrimony and hound's-tongue. Those who walk in the woods in late summer and fall are aware that plants have means of disseminating themselves (Fig.252). If it is impossible to identify the burs which one finds on clothing, the seeds may be planted and specimens of the plant may then be grown.

—174. What advantage is it to the plant to have its seeds widely dispersed? 175. What are the leading ways in which fruits and seeds are dispersed? 176. Name some explosive fruits. 177. Describe wind travelers. 178. What seeds are carried by birds? 179. Describe some bur with which you are familiar. 180. Are adhesive fruits usually dehiscent or indehiscent? 181. Do samaras grow on low or tall plants, as a rule? 182. Are the cotton fibers on the seed or on the fruit? 183. Name the ways in which the common weeds of your region are disseminated. 184. This lesson will suggest other ways in which