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mation. A tuber-forming stimulus is produced near the growing points 

 of the plant and moves basipetally. It is able to cross a graft union and 

 produce tubers in a noninduced plant. In plants with forked stems, 

 one half was given short periods and the other long. Tubers were 

 produced on that part of the stolon below the short-day branch. 



Reduction of growth of the axis, with the formation of leafy rosettes, 

 is favored by relatively short days. Thus Oenothera forms rosettes in the 

 fall and tall flowering stems in the next spring. Lettuce makes compact 

 heads in the short days of early spring but shoots upward in the longer 

 ones of June. 



In strawberries, long days increase leaf size and cell number (Arney, 

 1956). Ashby (1950b) reports that day-length also affects leaf shape in 

 Ipomoea, In 16-hour days, plants begin to flower at the fifteenth or 

 sixteenth node and lobing begins at the fifth to seventh node. Under 8-hour 

 days, however, flowering begins at the first node and lobing is almost 

 entirely suppressed. 



Gotz (1953) has made an intensive study of the effect of day-length 

 on the formation of plantlets on the leaves of three species of Bryo- 

 phyllum. In short days neither plantlets nor their primordia are formed 

 nor are flowers produced, and the leaves become somewhat more suc- 

 culent. Under long days, however, plantlets appear in abundance. The 

 effect of different day-lengths on plantlet production can be studied in 

 different leaves on the same plant. Neither grafting a scion from a long- 

 day plantlet-producing plant into a short-day plant, or injecting sap 

 from one, will induce the formation of these structures. 



The photoperiod also affects rooting of cuttings. Some species root 

 best under long days and others under short (Stoutemyer and Close, 

 1946). The photoperiod under which the stock plant has been growing 

 actually affects the rooting of cuttings taken from it more than does that 

 under which the cuttings themselves are grown (Pridham, 1942). 



As to leaf structure, Glimmer ( 1949 ) found that in Kalanchoe a change 

 in the photoperiod affects the thickness of the epidermal cell walls, the 

 size of the vein islets, and the form, size, and number of mesophyll cells 

 but that the number of stomata responds more slowly and the size and 

 form of the epidermal cells are unaffected. In the ten species she studied, 

 plants grown under short days had thicker leaves than under long ones, 

 and this was almost entirely because of greater size of the mesophyll 

 cells, which elongate at right angles to the surface of the lamina. They 

 also increase somewhat in width. If a single leaf on a Kalanchoe plant 

 growing under long days is itself exposed to short days, it grows con- 

 siderably thicker and changes its form somewhat. It is significant that 

 these changes are transmitted to other leaves directly above this one, 

 suggesting that a morphogenetic substance is involved. Detached and 



