vill 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 
Angiosperme, 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 G'netales, 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 Gymnosperme 
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 Gymnosperme, 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 Angiospermae, 
are as condensed as the special circumstances permit ; in each case, 
