EFFECT OF LIGHT ON PLANTS GARNER AND ALLARD. 585 



fall is as inevitable as is the death of the animal organism which 

 has lived out its allotted period of time. Until the problem of the 

 ultimate cause of senility is fully cleared up it maj 7 not be possible 

 to rescue permanently these annual plants from death, but it is at 

 least possible to prolong great!} 7 their life period, thus causing them 

 to behave as perennials. The relation of the length of day to the 

 two alternative phases of plant activity, viz, vegetative development 

 or growth and flowering and fruiting, has already been fully dis- 

 cussed, and it has been pointed out that a sufficiently wide change in 

 day-length may redirect the activities of the plant to the extent that 

 vegetative development is completely replaced by flowering and 

 fruiting. In the typical annual this change of activity is soon fol- 

 lowed by the death of the plant. On the other hand, it has been 

 pointed out that with a more moderate change in length of day there 

 may be only partial change from vegetative to flowering condition, 

 as in the " everblooming " condition. In this case death does not 

 necessarily follow, hence it appears that not only does relatively rapid 

 shortening of the days in the autumn and fall cause change from 

 the vegetative to the fruiting condition, but as a final result when this 

 change in activity is complete, may cause the death of the plant. With 

 no change in length of day these summer annuals would continue 

 to grow more or less indefinitely, so that senility in the organism as a 

 whole is deferred accordingly. If this be so, it seems possible 

 that the annual, having passed over into the condition of rapid de- 

 cline which follows flowering and fruiting as a result of the shorten- 

 ing of the days, could be rescued from approaching death by ex- 

 posure to a sufficiently increased day-length. As a matter of fact, 

 experiments in this direction have been entirety successful. 



It will be recalled that the Biloxi soy bean, which normally does 

 not begin to flower till September in the latitude of Washington, was 

 caused to blossom as early as June 16 by shortening the day-length to 

 only five hours. These same plants were restored to the full-day 

 length on June 20. The seed pods ripened rapidly, the leaves turned 

 yellow, and the plants at first appeared to be passing through the 

 usual changes leading up to final death. Eventually, however, new 

 shoots developed and the rejuvenated plants entered into a second 

 period of growth. The new shoots began to flower at exactly the same 

 time in September as did plants which had been growing all summer 

 in the field. Wild aster which had been forced to flower as early as 

 June 18 by shortening the day to only seven hours were restored to 

 the natural day-length on June 20. By July 20 new shoots had 

 appeared and the plants flowered the second time earty in September, 

 just as did control plants exposed throughout to the natural day- 

 length. Thus, perennials as well as annuals may be rejuvenated 

 through the influence of an increased day-length. Ragweed showed 



