CHAPTER XXIX 



THE DICOTYLEDONES: METACHLAMYDEAE 



The Metachlamydeae or Sympetalae are distinguished from the Archi- 

 chlamydeae by the more or less complete fusion of the petals to form a tubu- 

 lar corolla. This is achieved either by the lateral union of the petals or by 

 the development of their common base, by which the stamens are usually 

 also elevated on the side of the corolla tube. There seems little doubt that 

 the development of this tubular corolla results in a very close interplay 

 between insect pollinators and the plant. A tubular corolla with nectar 

 secreted only at its base prevents access to the nectar to all but long-tongued 

 insects, while at the same time it serves to protect the pollen from rain, and 

 confines the anthers to a position suitable for pollination when occasion 

 occurs. Though fundamentally the tubular corolla has apparently originated 

 similarly in all the families, in the more advanced members it has become 

 secondarily modified in a number of sharply contrasting ways. 



Associated with this sympetalous condition there has been a reduction 

 in the number of floral whorls as well as in the number of parts in each 

 whorl. The flowers are normally pentamerous, so far as the sepals and 

 petals are concerned, but there is frequently only a single whorl of stamens 

 and of the five stamens typically present one or more may be reduced to 

 staminodes or may be absent altogether. In the majority of the families 

 the ovary is composed of two carpels and the number of ovules in each 

 loculus is frequently small. The ovules themselves frequently possess only 

 a single integument. 



The flowers are arranged in various inflorescences, are seldom 

 solitary and show a marked tendency to zygomorphy. In the highest 

 families capitula are produced with a corresponding reduction in the size 

 of the individual florets. 



It is now generally agreed that the Metachlamydeae represent an advance 

 on the Archichlamydeae. They are a natural culmination of the evolu- 

 tionary trends exhibited within the latter families. It is however extremely 

 improbable that they are a monophyletic group. They represent rather the 

 ends of a number of separate evolutionary lines which arose in the Archi- 

 chlamydeae, all tending in the same direction. Whether each of the families 

 represents a single evolutionary series or whether each family is itself 

 polyphyletic is a matter of dispute. The true interrelationships of the 

 various families which comprise the Metachlamydeae have not yet been 

 settled, and their classification is still subject to considerable diversity of 

 opinion. 



1857 



