40 THE STRUCTURE OF FLOWERS. 



of the cords belonging to the sepals or petals, or from both ; 

 and similarly the dorsal cords of the carpels branch off from 

 the same stem as that of the sepals or petals, very rarely 

 from both at once. Simultaneously with the dorsal, two 

 marginal cords pass up directly into the placentas, having 

 originated in the same way; and, in so doing, the floral re- 

 ceptacle usually becomes extinct, and takes, as a rule, no 

 further part in the construction of the central portion of the 

 pistil. 



Starting, then, with these two fundamental sources of the 

 various arrangements of the parts of flowers, we may first 

 observe that of opposition or superposition and alternation, 

 the former, if represented by decussate pairs of appendages, 

 is the most primitive type. This is seen in many quaternary 

 flowers in which the sepals emerge in successively decus- 

 sating pairs. Such opposite leaves being foreshadowed in 

 the cotyledons of exogens. 



The next, or rather the first stage of differentiation is 

 seen in the spiral condition which obtains in many flowers, 

 mostly represented by the ^ and | types : thus, e.g., ^ repre- 

 sents the arrangement prevailing in petaloid Monocotyledons ; 

 and all pentamerous calyces issue in a quincuncial manner. 

 In Sahia, the petals follow continuously with the sepals in 

 the same spiral line, so that the first petal is superposed to 

 the first sepaL These whorls accordingly represent two 

 cycles of the f type, as seen above in Oaridella (p. 21). 



By far the commoner condition is to break up the spiral 

 into cycles, say of five parts each, and then to shift their 

 positions, so that they become alternate instead of superposed. 

 Now, such a decussate arrangement is usually described as a 

 fundamental law, not only governing opposite and verticillate 

 leaves, but floral whorls as well ; and particular stress is 

 laid upon the usual presence of the petaline whorl of carpels, 



