902 



ECOLOGY 



evidence for either. The exact cause of petalization is unknown, but 

 in many cases it appears to be inherent, double flowers usually being 

 regarded as sports or mutants, since they often may be reproduced by 

 seed as well as by cuttings. 1 In other cases, petalization clearly is due 

 to external factors, notably in a number of species in which plants whose 

 roots are infested with certain parasitic fungi (as Heterodera radicicola) 

 develop double flowers. Saponaria sometimes has double flowers 

 when the roots are infested with Fusarium. In the tulip, petalody 



is facilitated by 

 good nutrition, 

 especially if there 

 is an abundance 

 of nitrogenous sub- 

 stances in the soil. 

 In some cases 

 parasites cause 

 not only ordinary 

 doubling, but also 

 the development 

 of green foliage 

 leaves in place of 

 floral organs, the 

 phenomenon being 

 known as sepal- 

 ody or greening. 

 In parasitized in- 

 dividuals of Heli- 

 anthus strumosus, 

 greening is a common phenomenon, and not infrequently green foliage 

 leaves are intermingled with ligulate flowers in place of the usual disk 

 flowers. 



One of the most remarkable of all reproductive variations is that in 

 which flowers are replaced by bulbils, as in the wild garlic (Allium cana- 

 dense, fig. 1203), in whose umbels some primordia develop into flowers and 

 others into bulbils. Sometimes most or even all of the primordia develop 



1 Obviously, completely petaltzed flowers can be reproduced only by cuttings. In the 

 double petunia, which usually is propagated from seed, seeds are saved from flowers that 

 are almost double, and only 20 to 30 per cent of the progeny have double flowers. In the 

 composites complete doubling does not necessarily prevent seed production, double asters, 

 daisies, and sunflowers being raised regularly from seed. 



1201 



FIGS. 1201, 1202. Flowers of Narcissus, illustrating 

 petalody: 1201, an ordinary single flower with a six-parted 

 perianth (p) and a crown or corona (c)\ 1202, a double flower, 

 in which there is a considerable increase of the petaloid parts 

 at the expense of the stamens and carpels. 



