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LENGTH OF DAY INSTEAD OF TEMPERATURE 

 CONTROLS TIME OF FLOWERING AND FRUITING* 



For generations scientists have known that sunlight was 

 necessary for normal growth of most kinds of plants, and, al- 

 though the summer sun might occasionally become too hot, 

 they have understood that it could not cause any injury except 

 perhaps the injury due to burning. A recent discovery by W. 

 W. Garner and H. A. Allard, of the Bureau of Plant Industry, 

 United States Department of Agriculture, shows that, entirely 

 apart from any effect of burning, it is possible for plants to have 

 too much daylight or, in other words, too many hours of daylight 

 in comparison with the number of hours of darkness. Too long 

 a day as well as too short a day will prevent many kinds of plants 

 from ever reaching their stage of flowering and fruiting. 



Furthermore, the intensity of the light has very much less 

 significance upon the growth of the plant than has usually been 

 supposed. Greenhouse experiments prove that the flowering 

 and fruiting period of practically any plant can be made to take 

 place at any time of the year by darkening the greenhouse in the 

 morning and evening if the day is too long, or by lengthening 

 the day by artificial light if the day is too short. This new 

 theory of controlling flowering and fruiting of plants undoubtedly 

 will be used by florists and other greenhouse operators. For 

 example, violets bloom only during the comparatively short days 

 of spring; but if violet plants are covered with light-proof boxes 

 at night and not uncovered until the sun is about half an hour 

 high each morning during the summer time, violets can be forced 

 to bloom again in the summer. Spring flowers and spring crops 

 happen to be spring flowers and spring crops because the days 

 at the season of their flowering and fruiting have the proper 



* From a recent news bulletin of the U. S. Department of Agriculture. The 

 Journal of Agricultural Research recently contained an article of which this is a 

 popular account. The work is perhaps the most significant in recent ecological 

 research. An aifparent exception to the theory is the fact that the Lapland Rho- 

 dodendron, brought from the summit of Mt. Marcy to the Brooklyn Botanic 

 Garden, was kept all winter in the dark and flowered at Brooklyn six weeks before 

 the normal period above the timber line. In this case it flowered only about four 

 weeks after the winter covering was removed. — ED. 



