Un cours pratique en botanique : avec une référence particulière à ses roulements sur l'agriculture, l'économie et l'assainissement . conditions, il est clair à l'avantage d'une spécialité que ses graines devraient être dispersées aussi largement que possible, bothque les semis peuvent avoir beaucoup de place, et que les jeunes n'ont peut-être pas à puiser leur alimentation du sol alreadyépuisé par leurs parents. L'agriculteur reconnaît ce principe dans la rotation des cultures, parce qu'il sait que les cultures successives de la plante de thesame épuiseront bientôt le sol des substances i*e-quired pour sa nutrition, alors qu'ils peuvent le laisser plus riche inno

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A practical course in botany : with especial reference to its bearings on agriculture, economics, and sanitation . conditions, it is clearly to the advantage of a speciesthat its seeds should be dispersed as widely as possible, boththat the seedlings may have plenty of room, and that theymay not have to draw their nourishment from soil alreadyexhausted by their parents. The farmer recognizes this principle in the rotation ofcrops, because he knows thatsuccessive growths of thesame plant will soon exhaustthe soil of the substances i*e-quired for its nutrition, whilethey may leave it richer innourishment for a differentcrop. 25. Self-planting seeds.—Dispersal is not the onlyproblem the seed has to meet.The majority of seeds cannotgerminate well on top of theground, and must depend onvarious agencies for gettingunder the soil. Some of themdo this for themselves. Theseeds of the storks-bill, popularly known as filarees, havea sharp-pointed base and an auger-shaped appendage at theapex, ending in a projecting arm (the clock of the filaree)by which it is blown about by the wind with a whirling motion. Fig. 45. — Self-planting pod of peanut. THE SKKD 27 till it strikes a soft spot, when it begins at once to bore itsway into the ground. The common peanut is another exam-ple. The blossoms are borne under the leaves, near the baseof the stem, and as soon as the seeds begin to form, theflower stalks lengthen several inches, carrying the young podsdown to the ground, where they bore into the soil and ripentheir seeds. Practical Questions 1. Name the ten most troublesome weeds of your neighborhood. 2. What natural means of dispersal have they ? 3. Which of them owe their propagation to man ? 4. Are there any tumbleweeds in your neighborhood ? 5. Would you expect to find such weeds in a hilly or a well-woodedregion? (19, 23; Exp. 17.) 6. What situations are best fitted for their propagation? (19, 23;Exp. 17.) 7. Make a list of all the fruits and seeds you can think of that ar

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