Mar. j9,19*3 
Further Studies in Photoperiodism 
879 
The controls showed flower buds August 25 and open blossoms September 
9. The io-hour light exposure produced a decidedly darker shade of 
green in the foliage leaves than did the full summer day length. These 
short-day plants continued to grow slowly through the flowering period 
and were still green September 23. 
Seeds of Brazilian morning-glory (Ipomoea setosa L.) were planted 
June 8, and the seedlings had emerged from the ground on June 11. 
Exposed to 10 hours of light daily, the plants were showing flower buds 
July 5 and the first blossoms opened July 19. On the controls flower 
buds could be seen August 25 and the first blossoms opened September 9. 
The dwarf morning-glory, Convolvulus tricolor L., was planted June 8 
and had germinated June 11. Under a io-hour day the plants were 
unable to flower but remained green and continued to grow vigorously. 
The controls under the natural length of day were showing flower buds 
August i and first open blossoms August 11. After a period of flowering 
through August these plants reverted to the purely vegetative phase of 
activity after the first week in September, assuming a condition com¬ 
parable with that of the plants exposed to a io-hour day. 
Moonvine (C atonyction aculeatum (L.) House) planted June 8 had 
germinated June n. With 10 hours of light daily flower buds appeared 
July 15 and open blossoms August 5. On the controls flower buds could be 
seen July 23 and open blossoms August 16. 
Seeds of cypress vine (Quamoclit pennata (Desr.) Voigt) were planted 
June 8 and appeared above ground June 11. Under 10 hours of light 
daily, flower buds were showing June 2? and the first blossoms were 
open July 5. On the controls the first open blossoms appeared August 3. 
The short-day plants grew more rapidly and were darker in color than 
the controls exposed to the full day lenght. 
LONG-DAY AND SHORT-DAY PLANTS AND THE CRITICAL LENGTH OF DAY 
FOR FLOWERING 
As pointed out in the earlier paper and further illustrated in the 
preceding data, the plants studied tend to arrange themselves into two 
groups. One of these groups consists of species that are caused to 
flower by the action of short days while the other includes those species 
that are forced into flowering through the action of long days. For 
convenience the first named group are spoken of as short-day plants 
while the second group are designated as long-day plants. At first sight 
it would seem that these two groups of plants are diametrically opposed 
in their response to length of day, but detailed study of the two groups 
indicates that the difference is one of degree rather than of kind. In 
fact, classification into the two groups is more or less arbitrary. In the 
case of such plants as Cosmos and Bidens, flowering is inhibited by a 
daily light exposure much in excess of 12 hours’ duration, while vegetative 
development promptly gives way to flowering when the light period is 
reduced to 12 hours or less. These plants will flower even when receiving 
only a few hours of light daily; in other words, it is not possible to reduce 
the light duration sufficiently to prevent floweringwithout killing the plants. 
On the other hand, Solidago and Hibiscus are readily prevented from 
flowering by reducing the light period to something less than 12 hours, 
although under these conditions they may continue to live and may even 
continue development in certain directions. These plants are not inhib¬ 
ited from flowering by increasing the light duration beyond the normal. 
