l6-hour days, was no more than the small tree's. The one difference, a 

 contir.uous round of long days and short nights, kept the fast -growing 

 tree's phytochronne in active form at sufficient concentration to maintain 

 non-stop growth. 



Some additional kinds of young trees, though not all, evidently can 

 be induced to grow constantly on rightly managed photoperiodic schedules. 

 For nurserymen and plant breeders, such experiments promise methods 

 for speeding production of slow-growing stock. 



SOME ADDITIONAL CLUES AND FINDINGS 



Control Centers and Transmission 



In recent years, a start has been made toward locating control centers 

 from which phytochrome operates in plants, and learning the spread 

 effects to the growth change location. Evidence such as the following 

 indicates that both the control centers and the transmission system may 

 vary considerably with the kind of plant and its growth stage. 



Mature leaves are known to be control centers in which phytochronae 

 concentrates and gives flowering signals. In some cases, leaf signals 

 are transmitted to distant flowering shoots. Perhaps the first revelation 

 of a long-range network was the evidence that a single leaf on a soybean 

 plant could provide adequate stimulus to launch the whole plant's flowering 

 (page 6). Many experiments since have shown leaves doing such w^ork. 



In the pink flowering shrub Weigela, phytochrome in the uppermost 

 pair of leaves is apparently the control center for growth in each branch. 

 In one experiment, bud development in greenhouse Weigela was started 

 by providing summer-length nights, then halted by lengthening nights to 

 IZ hours --and then, the two terminal leaves on some branches were 

 removed and these branches resumed bud growth. The artificially induced 

 activity did not last long unless the plants were given longer days, re- 

 sembling their budding season. Yet the plants had reacted as if their leaf- 

 leaders were gone and had taken a halt signal with them. Removing lower 

 leaves had no such effect. 



Phytochrome at times operates a remarkably short-range network. 

 Mature toniatoes require red light so directly on each part to develop the 

 yellow in their skin that the phytochrome signals reach no more than two 

 millimeters (about a twenty-fifth of an inch) beyond an irradiated area. 



Tests have shown that the first 5 days of ripening, after tonnatoes 

 reach maturity, are the critical time in which they can launch skin 

 coloring. Light treatment started later has no skin coloring effect. So 

 little strength of light is needed that five -hundredths of a foot-candle-- 

 scarcely brighter than moonlight--is enough to bring out the skin color 

 if continued an hour a day for the ripening period of 10 to 14 days. 



Off-season market tomatoes do not develop the yellow pigment during 

 dark-room ripening and storage, so that the flesh under the transparent, 

 colorless skin shows through as pink, rather than red. The photoperiodic 

 tests indicate that lighting and turning tomatoes in commercial ripening 

 rooms would be necessary for uniformly bright color- -a process probably 

 too costly for wide commercial use. 



15 



