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 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 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 woolen 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? [Illustration: FIG. 253.--THE FRUITS OF THE CAT-TAIL ARE LOOSENED BY WIND AND WEATHER.] CHAPTER XXIII PHENOGAMS AND CRYPTOGAMS The plants thus far studied produce flowers; and the flowers produce =seeds= by means of which the plant is propagated. There are other plants, however, that produce no seeds, and these plants (including bacteria) are probably more numerous than the seed-bearing plants. These plants propagate by means of =spores=, _which are generative cells, usually simple, containing no embryo_. These spores are very small, and sometimes are not visible to the naked eye. [Illustration: FIG. 254.--CHRISTMAS FERN.--Dryopteris acrostichoides; known also as Aspidium.] [Illustration: FIG. 255.--FRUITING FROND OF CHRISTMAS FERN. Sori at _a_. One sorus with its indusium at _b_.]