Genetic studies on Cosmos bipinnatus V. Morphological development of flower-heads by the action of flower-type genes.(I)In single type malformed types, and sub-petaled type

Abstract

The present paper deals with the differences in morphogenesis between various flower-types of Cosmos with special reference to the expression of rays. Materials used in this experiment were single type (S type), malformed types (Mt, Vt, Np types) and sub-petaled type (MS type). Buds at successively higher levels of development could be gathered from the lower part to the top of the stens from each individual plant, and the differences in floral development were clarifcd by comparing the buds of the different flower-types at the same stage of development in materials dissected under magnification of moderate degrees. The development of S-type flower-head flollows the next sequence. The change from vegetative to reproductive activity in the apical meristem becomes evident after a short day treatment for one week. An individual capitate primordium is a broad and giobose protuberance from the bottom to the top of which appear the floret primordia like so many warts. The individual floret primordia become larger from the bottom to the top of the capitate primordium. Then the growth of the top part of each floret primordium begins to fall behind, and as the result a special cup-1ike structure appears. After a time, the structural difference between ray and disk florets becomes noticeable. In the development of disk florets, 5 growing points appear at the same interval along the margin of the cup-like primordium. With further growth, the five-parted petals with united base develop sympetalous actinomorphic structure of florets. In the development of rays, on the other hand, only the three growing points along the outside margin early on division and the other two along the inside are inhibited from making further divisions. Consequently, zygomorphic structure of florets with three petals united at the base follows. In the malformed types of flower-heads (ss genotype), the developmental pattern of disk florets fcllows the same pattern as that of S type. But the ontogeny of the rays of these types is quite different from that of S type. The developmental process at the earlier stages of the rays of these types resembles more those of disks of single or malformed types, that is, it follows the actinomorphic pattern of development. But later, as the floral development. advances, the growth rates between the outside and inside region of florets become increasingiy distinct, and at the fiowering stage the differences among three flower-types, Mt, Vt, Np, become distinct according to the degree in the developmental diffe