REPRODUCTION AND DISPERSAL 901 



tion. It was shown long ago that various zygomorphic flowers owe to 

 gravity their peculiar shape, since they become actinomorphic when its 

 influence is equalized. In Mimulus, zygomorphy is reduced in weak 

 illumination. Floral symmetry has been modified also by cutting con- 

 ductive strands that lead to the flowers and by otherwise changing the 

 nutritive conditions. Of particular interest are the experimental data 

 on cleistogamy, which involves marked change of form and structure, 

 especially in the corolla. In Stellaria, light is required for the opening 

 of the flowers, and in Linaria, flowers that usually are chasmogamous 

 in weak light are cleistogamous. In Lamium the vernal and autumnal 

 but not the estival flowers are cleistogamous. In Impatiens noli-tangere 

 the first flowers usually are cleistogamous and spurless, while the later 

 ones are chasmogamous and spurred; but if such a plant is transferred 

 to sterile sandy soil, only cleistogamous flowers are produced, indicating 

 that poor nutrition favors cleistogamy. Chasmogamous flowers may 

 be produced by Stellaria even in weak light, if the plants are supplied 

 with glucose. Parasites may induce cleistogamy; mildewed plants of 

 Impatiens produce only closed flowers, and Biscutella produces such 

 flowers, when the plants are attacked by cecidomyid insects. In Viola 

 mirabilis the primordia of chasmogamous flowers develop into closed 

 flowers in extreme conditions, as in dry, sterile soil and in a warm, 

 humid greenhouse, and in V. odorata the primordia of the cleistogamous 

 flowers develop into showy open flowers in dryish, sunny habitats. 



A most interesting floral modification is that seen in the so-called 

 double flowers (figs. 1201, 1202). Where the phenomenon is one of 

 the replacement of other floral organs (especially stamens and pistils) 

 by petals, it may be denominated petalody or petalization. 1 There 

 are varying degrees of petalody; for example, the buttercups, which 

 commonly have five petals, may have the number doubled or otherwise 

 increased even to complete petalization. In the white water lily (Cas- 

 talia), in which there are many petals disposed in several rows, the inner 

 members become smaller and narrower, and show all transitions to sta- 

 mens. Passing outward from the center, the stamen filaments become 

 broader and more petaloid, while the anthers gradually become effaced, 

 suggesting the possible origin of petals from stamens or of stamens from 

 petals; the first theory is the more reasonable, but there is no valid 



1 In the composites, however, doubling is due to the replacement not of stamens by 

 petals, but of disk flowers by ligulate flowers (as in double sunflowers and chrysanthe- 

 mums), so that one should speak of double heads rather than of double flowers. 



