Dispersal by Birds.—Seeds of berries and of other small fleshy fruits are carried far and wide by birds. The pulp is digested, but the seeds are not injured. Note how the cherries, raspberries, blackberries, June-berries, and others spring up in the fence rows, where the birds rest. Some berries and drupes persist far into winter, when they supply food to cedar birds, robins, and the winter birds. Red cedar is distributed by birds. Many of these pulpy 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 composite plants, allied to thistles, but the whole head, rather than the separate fruits, is transported. In some composite fruits the pappus takes the form of hooks and spines, as in the “Spanish bayonets” and “pitch-forks.” 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.
Fig. 252.—Stealing a Ride.
Suggestions.—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 fibres 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 seeds are transported. Nuts are buried by squirrels for food; but if they are not eaten, they may grow. The seeds of many plants are blown on the snow. The old stalks of weeds, standing through the winter, may serve to disseminate the plant. Seeds are carried by water down the streams and along shores. About woollen mills strange plants often spring up from seed brought in the fleeces. Sometimes the entire plant is rolled for miles before the winds. Such plants are “tumbleweeds.” Examples are Russian thistle, hair grass or tumblegrass (Panicum capillare), cyclone plant (Cycloloma platyphyllum), and white amaranth (Amarantus albus). About seaports strange plants are often found, having been introduced in the earth that is used in ships for ballast. These plants are usually known as “ballast plants.” Most of them do not persist long. 185. Plants are able to spread themselves by means of the great numbers of seeds that they produce. How many seeds may a given elm tree or apple tree or raspberry bush produce?
Fig. 253.—The Fruits of the Cat-tail are loosened by Wind and Weather.
CHAPTER XXIII
PHENOGAMS AND CRYPTOGAMS
Fig. 254.—Christmas Fern.—Dryopteris acrostichoides; known also as Aspidium.