VUl PREFACE. 



themselves impelled to extend the incidence of the term " flower," 

 and of the terms for various parts of the flower, so as to render 

 these applicable to all the phyla that are connected by homologies 

 within the sexual sphere of reproduction. As a logical consequence 

 even the sporangiophore of a horsetail and the fertile frond of a fern, 

 thus become " flowers." 



The value of this procedure as a means of correlating what might 

 otherwise appear wholly unrelated is doubtless great. But, even 

 where considerations of a practical nature may be neglected, we 

 soon come to realise that such endeavours to attain uniformity are 

 attended by a double disadvantage. They blunt the perception 

 of the very divergent lines along which the great plant-phyla have 

 been evolved ; they obscure the appreciation of the correspondingly 

 different structures in which the evolution of these phyla has 

 resulted. 



Those practical considerations which condition the preparation 

 of a technical work like the present flora cannot, however, be dis- 

 regarded here. For purposes of discrimination and of classification 

 it is of the utmost consequence that fundamental differentiations 

 of structure be expressed in suitable and distinctive terms. 



Here, therefore, the term " flower " and all that this word 

 connotes is confined, as it has been by Arber and Parkin, to the 

 AngiospermcB, a phylum characterised by the evolution of that 

 definite collection of organs with its normally cyclic structure, 

 its varied and specialised envelopes, its stamens, closed carpels, 

 styles and stigmas, and its peculiar type of fertilisation. The 

 ambiguous position of the Gnefales, and the eclectic treatment con- 

 sequently accorded to them in this work, have already been explained. 

 Leaving that class out of account, we recognise in the Gymnosperniw 

 a phylum within which the development of the reproductive system 

 has not gone beyond the evolution of structures, termed here cones 

 or strobiles, with usually scale-like leaves, abaxial (dorsal) pollen- 

 sacs, adaxial (ventral or marginal) openly exposed ovules, and a 

 corresponding mode of fertilisation. 



These features are more fully brought out in the definitions of 

 the Gymnospermce, and of the classes which that division includes, 

 prepared by Dr. Stapf for the conspectus of the orders contained in 

 this section of the " Flora of Tropical Africa." The definitions 

 of these gymnospermous classes, of necessity somewhat fuller than 

 the corresponding definitions of the classes of the Angiospermce, 

 are as condeiLsed as the special circumstances permit ; in each case, 



