56 
THE FERNS OF SOUTH AFRICA 
Although the Pteridophyta are thus scientifically dis- 
tinguished from all other plants (see also first chapter), its 
component groups differ from one another to a considerable 
degree, both in general appearance and in specific characters, 
and the student may find difficulty in recognising the common 
characters by which they are grouped together, and may also 
find difficulty in seeing a satisfactory basis of separation. This 
has been the experience of every botanist, and even now the 
highest authorities on systematic Pteridology approach the 
subject from very different standpoints, some working from 
the lower forms of plant-formation to the higher, and others 
vice versa. This alone merely involves simple inversion of the 
arrangement, but different conceptions of the homology and 
uses of certain organs lead to further differences of opinion. 
Prof. Bower’s scheme of arrangement (in so far as South 
African species are concerned) is : 
Cohort I. Lycopodiales. Sporangia arranged singly in the axils 
of subtending sporophylls. 
A. Eligulatae. Without a ligule, and homosporous. 
Lycopodium. 
B. Lignlatae. With a ligule on upper surface of leaf, and hetero- 
sporous. 
Selaginella and Isoetes. 
Cohort II. Equisetales. Sporangia borne on sporangiophores which 
are inserted directly upon the axis. Leaves in whorls. 
Equisetum. 
Cohort III. Sphenophyllales. Sporangia borne on sporangiophores. 
Leaves alternate. 
Psilotum. 
Cohort IV. Ophioglossales. Sporangia sunk in the tissues of a 
fertile spike. 
Oph iogl ossn m . 
Cohort V. Filicales. Sporangia in sori distributed over the under- 
surface of the leaves. 
A. Homosporous. (Sporangia and spores all alike.) 
i. Simplices. Sporangia of a sorus produced simultaneously. 
Marattiaceae , Osniundaceae , Schizaeaceae , Gleicheniaceae. 
2. Gradatae. (A definite succession of time and space in 
the production of sporangia.) 
Hymenophyllaceae , Cyatheaceae. 
