ABSTRACT
The genus Viola (Violaceae), with over 400 species, is native to both north and south temperate zones. Fifty-six Viola species are known to produce two distinct floral forms: (1) a showy, open or chasmogamous (CH) flower, and (2) a smaller, closed or cleistogamous (CL) flower. These morphological differences are environmentally induced. Cleistogamous flowers are produced under LD (14 to 17 hr) in all species studied. 1 - 5 Chasmogamous flowers are produced under SD (8 to 10 hr) in seven Viola species (Table 1). Viola sylvestris, V. arenaria, and V. lancifolia produce CH flowers only after a short period of low temperature. 3 No flowers are formed in SD alone, and a burst of CH flowering occurs after cold treatment even under LD. V. palustris produces occasional CH flowers under SD, but CH flowering is enhanced considerably by 8 weeks of vernalization at 4°C. 5 Under LD, this species responds like V. sylvestris, producing a burst of CH flowers after vernalization, followed by continuous CL flowering. Both cold treatment and SD are required for CH flowering in V. hirta. 3 The two flower types of Viola also represent differences in breeding systems. 6 The zygomorphic CH flower has several adaptations of form associated with insect pollination, and thus functions as an outcrosser. 7 , 8 All floral organs are reduced in the CL flower which selfs in the bud without opening.
