Fig. 82. Usnea barbata Web. Longitudinal section of filament and base of “soredial” branch × 40 (after Schwendener).
c. Soredia as Buds. Schwendener[489] has described soredia in the genus Usnea which give rise to new branches. Many of the species in that genus are plentifully sprinkled with the white powdery bodies. A short way back from the apex of the filament the separate soredia show a tendency to apical growth and might be regarded as groups of young plants still attached to the parent branch. One of these developing more quickly pushes the others aside and by continued growth fills up the soredial opening in the cortex with a plug of tissue; finally it forms a complete lateral branch. Schwendener calls them “soredial” branches ([Fig. 82]) to distinguish them from the others formed in the course of the normal development.
B. Soralia
In lichens of foliose and fruticose structure, and in a few crustaceous forms, the soredia are massed together into the compact bodies called soralia, and thus are confined to certain areas of the plant surface. The simpler soralia arise from the gonidial zone below the cortex by the active division of some of the algal cells. The hyphae, interlaced with the green cells, are thin-walled and are, as stated by Wainio[490], still in a meristematic condition; they are thus able readily to branch and to form new filaments which clasp the continually multiplying gonidia. This growth is in an upward or outward direction away from the medulla, and strong mechanical pressure is exerted by the increasing tissue on the overlying cortical layers. Finally the soredia force their way through to the surface at definite points. The cortex is thrown back and forms a margin round the soralium, though shreds of epidermal tissue remain for a time mixed with the powdery granules.
a. Form and Occurrence of Soralia. The term “soralium” was first applied only to the highly developed soredial structures considered by Acharius to be secondary apothecia; it is now employed for any circumscribed group of soredia.[491] The soralia vary in size and form and in position, according to the species on which they occur; these characters are constant enough to be of considerable diagnostic value. Within the single genus Parmelia, they are to be found as small round dots sprinkled over the surface of P. dubia; as elongate furrows irregularly placed on P. sulcata; as pearly excrescences at or near the margins of P. perlata, and as swollen tubercles at the tips of the lobes of P. physodes ([Fig. 83]). Their development is strongly influenced and furthered by shade and moisture, and, given such conditions in excess, they may coalesce and cover large patches of the thallus with a powdery coating, though only in those species that would have borne soredia in fairly normal conditions.
Soralia of definite form are of rather rare occurrence in crustaceous lichens, with the exception of the Pertusariaceae, where they are frequent, and some species of Lecanora and Placodium. They are known in only two hypophloeodal (subcortical) lichens, Arthonia pruinosa and Xylographa spilomatica. Among squamulose thalli they are typical of some Cladoniae, and also of Lecidea (Psora) ostreata, where they are produced on the upper surface towards the apex of the squamule.
Fig. 83. Parmelia physodes Ach. Thallus growing horizontally; soredia on the ends of the lobes (S. H., Photo.).
b. Position of soraliferous Lobes. According to observations made by Bitter[492], the occurrence of soralia on one lobe or another may depend to a considerable extent on the orientation of the thallus. He cites the variability in habit of the familiar lichen, Parmelia physodes and its various forms, which grow on trees or on soil. In the horizontal thalli there is much less tendency to soredial formation, and the soredia that arise are generally confined to branching lobes on the older parts of the thallus.
That type of growth is in marked contrast with the thallus obliged to take a vertical direction as on a tree. In such a case the lobes, growing downward from the point of origin, form soralia at their tips at an early stage ([Fig. 84]). The lateral lobes, and especially those that lie close to the substratum, are the next to become soraliate. Similar observations have been made on the soraliferous lobes of Cetraria pinastri. The cause is probably due to the greater excess of moisture draining downwards to the lower parts of the thallus. The lobes that bear the soralia are generally narrower than the others and are very frequently raised from contact with the substratum. They tend to grow out from the thallus in an upright direction and then to turn backwards at the tip, so that the opening of the soralium is directed downwards. Bitter says that the cause of this change in direction is not clear, though possibly on teleological reasoning it is of advantage that the opening of the soralium should be protected from direct rainfall. The opening lies midway between the upper and lower cortex, and the upper tissue in these capitate soralia continues to grow and to form an arched helmet or hood-covering which serves further to protect the soralium.