Botanical Gazette, 103(4): 698-709 (June 1942)
Photoperiodic After-Effects in Six Composites
Victor A. Greulach

Photoperiodic after-effects were discovered in Cosmos bipinnatus by Garner and Allard (3) and have since been observed in several other species of composites. Biddulph (1) found that when plants of the Klondike variety of C. sulphureus were transferred from short to long photoperiods, anomalies developed, including change from opposite to spiral phyllotaxy, elongation and foliation of bracts, abortive buds, twin flower heads, and elongated internodes in the region of involucral bracts. Garner and Allard did not describe such structures for C. bipinnatus. Murneek (7,8) found that when Rudbeckia plants were transferred from long to 7-hour photoperiods some plants failed to bloom; some formed the usual type of flower heads (except that these were either sessile or borne on short peduncles, the plants remaining in the rosette condition): and some bore "vegetative flowers" with green petals and more or less vegetative stamdens and pistils.

In an effort to determine whether similar photoperiodic after-effects occur in certain other species of composites, experiments were conducted on Cosmos bipinnatus Cav., C. sulphureus Cav. (Orange Flare variety), Rudbeckia hirta L., Matricaria parthenoides Desf., Centaurea cyanus L., and Coreopsis tinctoria Nutt. Both species of Cosmos were found by Garner and Allard (3) to be short-day plants. However, the Orange Flare variety of C. sulphureus, unlike the Klondike variety, is not strictly a short-day plant. Roberts and Struckmeyer (12) reported that it acted as a short-day plant in cool temperatures and as an intermediate or neutral plant at higher temperatures, blooming under both long and short photoperiods. The species of Rudbeckia have generally been classed as long-day plants, but Roberts and Struckmeyer (11) found that under low night temperatures R. laciniata formed flower buds after a relatively long time. The other three  species were reported by Poesch and Laurie (9) to be long-day plants.


FIG.1.—Representative floral modifications in Cosmos bipinnatus, August 1. Numbers refer to number of short photoperiods to which plants had been exposed. From left to right the specimens represent: flower head with elongated bracts (one separated from the others by stem elongation); two with elongated bracts in a whorl some distance below the head, bracts of first one being incised; bracts elongated but not separated from head; bracts elongated and scattered, with reversion to vegetative growth; bracts only slightly elongated and in usual position