PHOTOPERIODISM . 691 



day. This has been shown to hold truo for a wide variety of plants and 

 in regions representing a very wide range in latitude. Adequate vegeta- 

 tive growth, of course, is a prerequisite for extensive tuber development, 

 and inasmuch as general growth usually is much restricted by a decidedly 

 short light period, it commonly occurs that maximum absolute yield of 

 tubers is obtained with only a moderately short or intermediate length 

 of day or more rarely with a long day. On the other hand, bulb forma- 

 tion, at least in the case of the onion, is induced by a long day, though 

 there are distinct varietal differences as to the most favorable daylight 

 period for this response (47). 



SENESCENCE, DORMANCY, AND RELATED PHENOMENA 



With suitable light periods both the long-day and short-day types 

 of plants may maintain vegetative activity for very long periods of time 

 without appearance, or definite evidence, of senility in the organism as a 

 whole. In annuals, exposure to optimal light periods for flowering and 

 fruiting tends to induce rapid senescence and death. Similarly certain 

 light periods may cause perennials to enter into a state of dormancy. 

 Periods of light which are only moderately suboptimum may permit 

 limited reproductive activity, and in this case rapid senescence may 

 be limited to certain plant organs or parts, while the organism as a whole 

 is able to continue activity. Again, it has been shown (22) that typical 

 annuals as well as perennials in which intensive reproduction has been 

 initiated by a favorable day length may be rejuvenated by transfer to a 

 length of day promoting vegetative activity. Such typical annuals as 

 soy beans and the perennial Aster linariifolius were made to complete 

 two cycles of alternate vegetative and reproductive activity within a 

 period of 4 months. Schaffner (63) also was able to effect rejuvenation 

 in Cannabis saliva by applying continuous illumination to plants which 

 previously had been exposed to the natural short day of winter and had 

 flowered. If the plants come into flower in spring when the day is rapidly 

 lengthening, they may rejuvenate naturally, and after rejuvenation there 

 is a succession of leaf forms corresponding to those which develop in 

 juvenile plants from seed. 



In tests by Garner and Allard (23) cultures of Liriodendron tulipifera 

 were transferred to the greenhouse in September and one series received 

 weak supplementary illumination from sunset till midnight. With 

 natural light alone the plants soon lost their leaves by abscission and 

 remained dormant through the winter. Under the long-day conditions 

 there was prompt renewal of active growth with abundant development 

 of new leaves. There was no definite period of leaf fall and as individual 

 leaves died the petioles remained firmly attached to the stem. In 

 Rhus glabra L. there was no new top growth with the long day but there 

 was no leaf fall, the leaves retained a dark green color, and eventually 



