20 MORPHOLOGY OF AXGIOSPERMS 



which the primordia of the petals appear after those of the 

 stamens, and -each apparently from the dorsal surface of a young 

 stamen. The conclusion that the so-called petals of this family- 

 are not morphologically petals, but stamineal outgrowths, is 

 unnecessary, since the phenomenon can be more logically in- 

 terpreted as a case in which the primordia of stamen and petal 

 have a common origin, entirely analogous to the sympetalous 

 corolla with stamens " inserted on its tube," but in which the 

 separate primordia of the petals have been delayed in their 

 appearance. Such examples as those enumerated above simply 

 serve to emphasize the desirability of a more serious and sys- 

 tematic investigation of the whole subject. 



In the case of members of a single cycle, it is a question 

 whether their primordia ever appear simultaneously, although 

 they may appear in rapid succession. In zygomorphic flowers, 

 however, the succession is probably always evident. For ex- 

 ample, Goebel * cites the case of the papilionaceous Legumino- 

 sae, in which the anterior median sepal first appears, then those 

 to the right and left of it simultaneously, and finallv the two 

 obliquely posterior ones; but before these last are evident the 

 two obliquely anterior petals appear, and after them the other 

 three in the same order as the corresponding sepals. This 

 succession proceeds right and left from the anterior member 

 to the posterior. In other known cases, however, as in Reseda, 

 according to Payer, the succession is right and left from the 

 posterior member to the anterior. 



It must also be noted that a meristematic zone giving rise 

 to a set of members may add to the set later or even duplicate 

 it, giving rise to the well-known interposition of new members 

 or new sets. For example, it is stated that among the Gera- 

 niaeeae, Rutaceae, and Zygophvllaceae a new cycle of five sta- 

 mens is interposed among the five already formed ; and that 

 in Aceraceae and Sapindaceae two to four stamens are inter- 

 calated in the complete cycle of five previously formed. This 

 later interposition of new sets or new members has been re- 

 corded chiefly for stamens, and is a prolific source of inter- 

 ference with the " symmetry " of numbers. 



All seed-bearing plants are necessarily dioecious since they 



* Goebel, 0. Outlines of Classification and Special Morphology. English 

 translation. 1887. p. 424. 



