. Éléments d'écologie. L'écologie. 226 la lumière. Fig. 6.22. Le contrôle de la floraison et de type de la croissance végétative par la longueur du jour dans une culture en jardin plante à jours courts typique. Les expositions journalières (en heures) pour la lumière sont indiqués sur chaque conteneur ; C  = la photopériode naturelle. Photo prise en juillet. La floraison a eu lieu comme suit : 10 hr, le 6 juillet ; 12 lir, 14 octobre ; 13 h 19 octobre ; 14 h, le 31 octobre, les 24, 31 octobre. (Allard et Garner, 1940.) la croissance avant la floraison. Au début du printemps des fleurs sur le sol de la forêt de feuillus sont en mesure de tirer parti de la grande quantité de lumière disponible avant d'e

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. Elements of ecology. Ecology. 226 Light. Fig. 6.22. Control of flowering and of type of vegetative growth by daylength in garden chrysanthemum, a typical short-day plant. The daily exposures (in hours) to light are indicated on each container; C = natural daylength. Photo taken in July. Flowering occurred as follows: 10 hr, July 6; 12 lir, Oct. 14; 13 hr, Oct. 19; 14 hr, Oct. 31; natural day, Oct. 31. (Allard and Garner, 1940.) growth before flowering. Early spring flowers growing on the floor of the deciduous forest are able to take advantage of the great amount of light available before the tree leaves are fully developed. Since short days occur later in the summer at higher latitudes, the northward distribution of short-day plants may be stopped by the lack of sufficient time for the ripening and hardening of the seed be- tween the occurrene of the critical daylength and the occurrence of a killing frost that terminates the growing season. Ragweed, for example, which flowers when the days have shortened to 14^ hours, will begin its reproductive phase about the first of July in Virginia, and hay-fever sufferers notice its pollen in the air by the middle of August. In northern Vermont the summer days are not reduced to 14^ hours until after the first of August, with the result that seed formation is not completed before cold weather, and consequently ragweed is unable to establish itself in abundance at this latitude. The distribution of long-day plants toward the equator is limited by the reverse action of this aspect of the light factor. Species of Sedum that require a daylength of 16 hours or more will flower abundantly in Vermont but will not bloom in Virginia (Naylor, 1952). Plant species in which flowering is not controlled by daylength also occur at all latitudes, but all plants growing at high latitudes must be able to tolerate long days and short nights since these light condi- tions exist during the growing season, and tropical plants must be. Please no

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