Gelatinous lichens

In some gelatinous lichens, for example in Leptogium, the hyphae are compacted at the surface of the thallus into a pseudoparenchymatous layer which is only one cell thick. Seen from above, this tissue consists either of isodiametric cells pressed together in an unbroken layer or of loosely organized irregular cells (Fig. 22). 

The influence of the ascocarp on the anatomy of the thallus is most conspicuous in gelatinized lichens. The thallus of these species seems to be too soft to bear the fruiting bodies and therefore either secondary supporting... 

In our experiments (Blum, 1964), the time required for saturation for most of the investigated fruticose and foliose lichens was 2-4 minutes, for gelatinous lichens Collema cristatum and C flaccidum) about 6 minutes, and for Dermatocarpon vellereum and D, miniatum about 9 minutes. The crustose, wandering, desert lichens Aspicilia esculenta and A, fruticulosa achieved saturation in 17 and 26 minutes, respectively. 

Some lichens, such as Collema, which have Nostoc as a photobiont, do not form a well-organised thallus (Fig. 1.1). In these cases, fungal hyphae grow inside the thick gelatinous sheaths of the photobiont, which make up much of the thaUus (Paracer and Ahmadijan 2000). 

In some fruticose lichens, such as Usnea which has a radial arrangement of the tissues, a central axial strand can be distinguished internal to the medulla (Fig. 46). The structure of the central axis is dense and consists of paraplectenchymatous or prosoplectenchymatous tissue giving considerable tensile or skeletal strength to the thallus. In other genera, i.e., Alectoria, Cladonia, and Ramalina, the central axis is absent. Its place can be taken by a central hollow or by gelatinous or spongy tissues. In Letharia a central cord is formed by fusion of several smaller strands (Fig. 29). 

All tissue types can build a lichen cortex. Occasionally, two different tissues form a cortex which then appears as a two-layered structure. For example, in Ramalina siliquosa the outer part of the cortex is formed by a few parallel to reticulately orientated hyphae, connected by anatomoses. The rest of the thick cortex is formed of hyphae with gelatinized walls with a fastigiate arrangement. The hyphae of the two tissues lie at right angles to one another (Fig. 31). 

The ascal wall in lichens, fundamentally conforming to the general type (Chadefaud, 1942, 1960, 1969), consists of two layers, both complex (1) an external firm layer, the exoascus, often covered with the gelatin mentioned above, and (2) an internal layer, the endoascus, sometimes thick and basically double. An internal membrane can be added to these layers, in connection with the plasmalemma of the ascus. 

Particularly, the SEM gives an excellent view of hyphal distribution. The cortex can be discerned as a plectenchyma (Fig. 19) or as hyphae embedded in a thick gelatinous substance (compare Figs. 13 and 18). Hyphal orientation in the medulla is clearly evident. Numerous small and tightly interwoven hyphae can be seen as well as hyphae which are thicker in diameter but fewer in number. Within the thalli the distribution of crystals of lichen acids also can be studied with the SEM. The crystals are attached directly to the walls (Figs. 26 and 28) or they are scattered between the symbionts (Fig. 27). 

Most lichens, except gelatinous forms, accumulate less water during saturation than other cryptogamic plants (fungi, algae, and mosses). Water accumulation in the thallus is very uneven. Different parts of the thallus and its layers contain different amounts of water. There are no data about the water content of mycobiont and phycobiont cytoplasm, but it is considered to be small. 

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