90 MORPHOLOGY OF THE ANGIOSPERMS 



has undoubtedly arisen in two ways. It represents modified stamens in 

 most families; in some families, it represents the distal part of a primi- 

 tive unspecialized perianth. The effect of reduction of the perianth is 

 primarily on the corolla; the petals are reduced in prominence, to small 

 nonpetaloid organs, often easily overlooked, or to complete external 

 loss. The "apetalous" condition has commonly been considered an early 

 stage in flower development, one in which the perianth is still in primi- 

 tive form. But, in many taxa, the apetalous condition has been shown to 

 represent a high stage in perianth specialization rather than a low one 

 — Salicaceae, Betulaceae, Juglandaceae, Urticaceae, Aceraceae, Plata- 

 naceae, Proteaceae, Fraxiniis. 



The Fertile Appendages. The fertile appendages, also of leaf rank, are 

 of two types: microsporophylls (stamens) , which bear microsporangia; 

 and megasporophylls (carpels), which bear megasporangia. The stamens 

 constitute the anclroecium; the carpels, the gijnoccium. (The term pistil 

 is applied to a unit of the gynoecium: to a single carpel when the 

 carpels are free from one another; to a group of carpels when they are 

 fused to one another.) Where the flower has only one kind of sporo- 

 phyll, it is unisexual; where both are present, it is bisexual. If flowers 

 of both unisexual kinds are borne on the same plant, the taxon is 

 monoecious; if staminate and pistillate flowers are borne on separate 

 plants, the taxon is dioecious. 



The flower varies greatly in number of kinds of parts, from four to 

 one; in number of organs of a kind, from many to one; in extent of 

 fusion of organs to one another, from complete absence of fusion to 

 connation and adnation in extreme form; in elaboration of organs in 

 size and form. The range in flower form and structure is very great — 

 from the (superficially) simplest type, in which the flower consists of 

 a single sporophyll on a receptacle, as in the staminate flowers of 

 Euphorbia (Fig. 36A), to those with all kinds of appendages present, 

 extensively fused together and elaborate in form. Variety in form is 

 recognized as largely the result of reduction and of fusion. (Closely 

 similar conditions are seen in the cones of the conifers.) 



Broad comparative studies of flower structure show lines of spe- 

 cialization and, in general, the sequence in which changes have oc- 

 curred. From these, the more primitive structure is determined and 

 the course of phylogenetic modification can be outlined. The structure 

 of the flower has been the chief basis for the classifications, artificial 

 and natural, of angiosperms. And, although it is now recognized diat 

 evidence of natural relationships must come from all parts of the plant, 

 vegetative and reproductive, and from all fields of plant study, flower 

 structure provides a foundation upon which theories of evolutioriary 

 relationships can be based. 



