The Fruticose Species of Usnea Subgenus Usnea in East Africa

212 THE LICHENOLOGIST Vol. 11
non-pigmented groups. (The most exceptional case in the pigmented groups,
namely, the primary species with pigmented medulla (Usnea subflorida), with three
chemical races, had two of those represented at only three out of the 22 localities
from which collections of this species were made.)
The three non-pigmented groups were then examined in order to delimit the
species within them, and they are discussed in turn below.
In the group of non-pigmented primary species Usnea picta could be separated
out as a robust species with stiff branches and many tubercles rather than papillae.
The stiffness of the branches appears to be due partly to the thickness of the cortex,
and this character was therefore compared in plants assigned respectively to U.
picta, U. submollis, and U. nodulosa. (Both the last two species have robust morphotypes
that might be compared with U. picta, whereas U. complanata is a slenderer
species and U. albontaculata has other differences, discussed below.) Five specimens
of U. picta, U. submollis, and U. nodulosa were chosen of comparable size, three
main branches from each were sectioned, and the width of the cortex was measured
microscopically to the nearest 0-05 mm. Since both inner and outer surfaces of the
cortex are ridged and nodular, the width was measured so far as possible at points
without these irregularities. The results are shown in Table 3. Though the differences
are slight they are fairly consistent and when tested further were found to be
taxonomically helpful if interpreted as follows: the cortex of U. picta is usually
0-1 mm thick or more, while in the other species it is usually less.
TABLE 3. Mean width of cortex (mm) of three branches from five specimens of species indicated
Species
1
2
Specimen
3
4
5
Grand
mean
U. picta
U. submollis
U. nodulosa
010
010
008
013
009
008
011
008
010
011
009
009
009
009
008
011
009
009
Like Usnea picta, U. nodulosa has tubercles rather than papillae; they are often
elongate or effigurate as well as orbicular. However, U. nodulosa has branches that
are more pliant owing to their thinner cortex, more often develop laterally instead of
by isotomic dichotomy, and are more often fusiform than tapered.
The other species in this group are characterized by having papillae and small
nodular tubercles rather than the larger, linear or effigurate tubercles of Usnea picta
and U. nodulosa. These species are Usnea complanata, U. submollis, and U. albomaculata.
(U. intumescens Mot., a doubtful species in the U. submollis aggregate,
also comes into this group but is excluded from separate consideration: see under
U. albomaculata below.) It appeared from our material that the frequency of spinules
varied inversely with that of papillae or tubercles. Some specimens are intermediate
in this respect, and decisions on them were doubtful. However, after
examining the types of these three species we assigned our specimens to two main
groups. The first group, with the frequency of spinules high and of papillae low,
was taken to represent U. complanata. The second group, with the frequency of
spinules low and of papillae high, was taken to represent U. submollis and U.
albomaculata.