Lichens

(2) Radiate lichens, upright fruticose forms (fig. 6), start from a rooting base ; the fronds are exposed to sun and air on all sides and the structure is alike round the whole surface. The cor tex is of several types :—of decomposed cells, of densely packed, fastigiate hyphae or of longitudinal, thick-walled fibres. All these variations give strength and pliancy to the fronds. Other strength ening structures are the "sclerotic" fibres which extend up the frond either just with in the gonidial zone as in Ramalina or, as in Usnea, in one strong chondroid central strand of great toughness and strength. Growth in radiate lichens is apical or inter calary. Fronds of Rocellae, Ramalinae, etc., reach a height of several inches ; pen dulous lichens often grow in long stream ers : a length of ten metres has been re corded for Usnea longissima in the tropics.

In Cladoniae, the cup lichens, there is a double thallus : leaflet squamules of stratose structure, with upright stalks or podetia, frequently widening to a cup or scyphus at the top (fig. 7). The podetium becomes hollow in time but is firm and strong owing to the sclerotic fibres that line the central tube. Fructifications are borne on the tips of the podetia, or on the edges of the scyphus, which seems to indicate that the podetium originated as a fruit stalk. There is great variety of form, texture and colour in each of these main groups.

Special Lichen Structures.

Cyphellae. There are no sto mata in lichens, but ample provision is made for aeration and for gaseous exchange. Definite aeration structures (fig. 8), cyphellae or pseudo-cyphellae pierce the thick under-cortex of the Sticta ceae. They are small and cup-like, in cyphellae with an overarching margin; the base rests on the medulla and the cup is filled with small loose cells. Pseudo-cyphellae lack the cup margin. In other lichens there occur dot-like openings; the lines between the areolae admit air, or the surface is seamed by cracks and delicate reticu lations; soredial openings are present in many species, and in Parmelia exasperate there are true breathing-pores—minute cone like outgrowths, open at the summit.

Cephalodia. These occur as excrescences on the thallus of Archilichens (with bright green gonidia), and always contain blue green cells, mostly Nostoc or Scytonema. They are small bodies of various form and size from the minute pustules on the surface of Peltigera aplitliosa (fig. 9) to the coral

loid masses on Lobaria laciniata. Blue green cells alight by chance on the thallus and the cortical hairs grow out and gradu ally form a cortex round them. In a few instances there are groups of blue-green cells which are absorbed into the thallus by the under-surface, and a layer of blue green algae, below the normal bright green zone in Solorina crocea, also rank as cephalodia. These alien bodies seem to in dicate an ancestral association of the particular lichen with blue-green gonidia, the power to combine having persisted along with the presumably more recent symbiosis with the bright-green alga.

Soredia. As already indicated these are minute portions (hyphae and gonidia) that break away from the parent thallus and serve for propagation of species. The simplest types are diffuse soredia that in certain conditions of shade or mois ture cover the surface of the plant. More defined forms are termed soralia which arise by the upward push of hyphae in the gonidial zone, and emerge as roundish or oblong bodies packed with soredial granules. These multiply and, as they become detached, are easily dispersed. Soralia are more or less specifically constant in form and size and in their position on the surface or margins of the lobes.

Isidia.

Many lichens are rough on the surface owing to small outgrowths called isidia. So noticeable are they that Acharius established the genus Isidium to include isidiose lichens. They are cortical structures and begin generally as a small swelling or wart : they are upward extensions of the tissues and the cortex is continuous over their surface; sometimes they are darker in colour than the normal thallus as in Evernia furfuracea.

Structure of Basidiolichens.—There are three genera re corded in this group of tropical lichens : Cora, Corella and Dic tyonema. The gonidia, Chroococcus or Scytonema are Myxo .

. ,,,}4 phyceae. Cora and Dictyonema 7 „,,J;,:::.:::",. ..:.:„.,,..:.,:,,.., - are of a thin bracket-like form; , ' 4i they grow on the trunks and ,,...:,- • , - ',,:.,,,-, branches of trees, very rarely on ? ., W #. the ground, and are attached by (;,,,,,E: rhizinae. No proper cortex is formed, but in Cora the hyphae