The Fruticose Species of Usnea Subgenus Usnea in East Africa

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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.
1979 Usnea in East Africa—Swinscow & Krog 213 Separated on this basis, the two groups remained mixed chemically, but when they were examined in relation to the altitude at which they were collected a distinct difference was found (Table 4). In making this comparison we included only those specimens collected in East Africa and excluded those collected by Stauffer on the Virunga Volcanoes, as they were all or nearly all just outside our area. The montane zone begins at around 2000-2500 m altitude. Table 4 shows that 34 out of 39 localities (87%) for the Usnea complanata aggregate were below this altitude, whereas for the U. suhmollis aggregate only six out of 49 localities (12%) were below it. TABLE 4. Numbers of collections at separate localities of U. complanata and U. submollis aggregates analysed by chemotype and altitude Chemotype U. complanata aggr. psoromic salazinic norstictic stictic U. submollis aggr. alectorialic +_ salazinic norstictic ± salazinic psoromic stictic fumarprotocetraric — — — — — — — — 1 2 Altitude (m) A 1000- 9 3 — 5 1 1 — 1500- 10 3 3 1 4 — — 2000- — 3 \ — 6 1 1 2500- — 1 — — 9 1 4 3000- — — — — 10 2 2 3500 + — — —— 3 1 — — — Further 1 division of the Usnea complanata aggregate was considered but rejected. A number of morphotypes were recognizable, however, and details are reported under U. complanata in the systematic section below. In the Usnea suhmollis aggregate, though several taxa with similar morphology and chemistry are reduced to synonymy, U. albomaculata is retained as separate for the time being. But whether it is specifically distinct from U. submollis is doubtful. Its type differs from the latter species in having some scattered large pseudocyphellae on the main branches, extensive efflorescence of medullary tissue from cracks and articulations, a black base to the thallus, and psoromic acid. At least some of the pseudocyphellae have been formed by a branch or fibril breaking off, enlargement of the cortical aperture thus left, and extrusion into it of medullary tissue, with the broken end of the axis of the branch or fibril remaining in the centre. These cortical openings might therefore more aptly be described as scars than pseudocyphellae, though they seem capable of serving the function of gas exchange. The extent to which such pseudocyphellae and/or medullary extrusions from cracks and joints are developed varies over a continuous range from absence to abundance. The presence or absence of a black base may be difficult to determine. Even when well developed, the blackening does not extend more than about 5 mm above the
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The Fruticose Species of Usnea Subgenus Usnea in East Africa

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