Thallus medulla

Two vanadium bromoperoxidases that differ in carbohydrate content [26,33] have been isolated from A. nodosum. The most abundant bromoperoxidase, V-BrPO-I, was found in the thallus, and the other bromoperoxidase, V-BrPO-II, was reported to be present on the thallus surface [26], A previous report also concluded that V-BrPO is present in two different locations of A. nodosum, one in the cell walls of the transitional region between the cortex and medulla of the thallus and the other in the cell wall of the thallus surface [34], More recent experiments demonstrate that vanadium-dependent bromoperoxidase activity is present in both the cortical and surface protoplasts of M. pyrifera [35], L. saccharina, and L. digitata [36], The biosynthesis of V-BrPO in the protoplasts of L. saccharina has been shown using [35S]-methionine [36], The vanadium bromoperoxidases are all acidic proteins [26] with very similar amino acid compositions [37], V-BrPO (A. nodosum) has been crystallized, although refined structural data have not been reported yet [38], A different isolation procedure, based primarily on a two-phase extraction system, has been described [39,40], This procedure works well for certain types of algae (e.g., Laminaria) but not for the isolation of V-BrPO from A. nodosum, the principal source of V-BrPO for the mechanistic studies. 

Spot test This test has been used universally as rapid, nrm-specific means for detecting the presence of certain unspecified lichen substances. This test is most convenient and simple to perform, even under field conditimis. However, this is only a preliminary step in the process of identification of lichens or its substances. In order to identify accurately the secondary metabolite present in the lichen thaUus, one has to perform more sensitive test such as TLC or HPLC. Spot test is carried out by placing a small drop of reagent on the lichen thallus, either directly on the upper surface (cortex) or on the medulla. In the later case, the cortex is scraped or superficially cut with the help of a blade. The reagents used are 10 % aqueous KOH solution (K), saturated aqueous solution of bleaching powder (NaOCla) or calcium hypochlorite (Ca(OCl)2) (called as Q and 5 % alcoholic... 

Most lichens are more complex in structure. The algae are restricted to a particular layer in the thallus and besides the algal zone there is at least one other defined layer, the medulla, which contains no algae. Other layers, a cortex for example, may also be developed. These thalli with a stratified organization are called heteromerous (Fig. 93). 

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). 

The layers of the thallus—upper cortex, algal layer, medulla, and lower cortex—are more or less present in all heteromerous lichens. Those lichens with a radially organized thallus are no exception. Here the only difference is... 

In most lichens with two algal symbionts the blue-green algae are not distributed within the thallus but lie in special, delimited, swollen parts of the thallus. These structures, sited either in the medulla or on the upper or lower surfaces of the thallus according to the species, are called cephalodia. 

Internal cephalodia are formed in a similar manner. Here, too, the algae are trapped by the hyphae of the cortex (Jordan, 1970 Jordan and Rickson, 1971 Moreau, 1928). They are enmeshed by a thick layer of fungal cells and pressed into the thallus, where the cephalodium is eventually formed (Fig. 126). The enmeshing hyphae form a bundle which pushes the algae inside. Usually, the algae penetrate via the lower surface of the thallus but sometimes they seem to enter via the upper surface of the lichen. If this happens the cephalodium lies above the algal layer, which is pushed deeper into the medulla (Fig. 127). 

Many of the types of tissues and cells found in the cortex and medulla of lichens are present also in the fruiting body, especially in the exciple. These structures are treated in Chapter 2 but there are some examples of the way these tissue types may become altered in the anatomy of the thallus adjoining the ascocarp. 

Figs. 18-19. Fig. 18, cross section of one-half the thallus of Usnea ceratina showing outer cortex (Co), algal layer (A), medulla (M), and the central cord (Z) Fig. 19, cross section of Peltigera rufescens thallus showing plectenchyma (PL), algal layer (A), and the thick interwoven medullary hyphae (M). Material air-dried. 

In Lobaria laetivirens and the thallus of Lobaria amplissima inhibition was brought about only by the presence of ribitol in the medium and was substantial only after 24 hours. glucose had no effect of release into the medium although a reduction in total fixation was noted. The rate at which inhibition could be affected in these lichens was also reflected in the rate at which moved from the algal layer to the medulla as measured by... 

Fig. 16. Special kinds of adaptations to the desert environment in South Africa (A) Lecidea crystallifera cross section of the thallus. P, cortex cone Pi, pigment stratum Go, chains of phycobionts Py, pycnoconidia O, aperture of the pycnoconidia M, medulla Rhi, rhizinic hyphae Rz, rhizoidal string. (B) Buellia sp. on a quartz block. I, general view of the thallus on the quartz (natural size) II, cross section of the thallus III, model of the course of light to the phycobionts of the inverse thallus. (Ap, apothecium S, hyphae with soil particles An, groups of phycobionts Go, phycobiont layer Q, quartz L, incident light B, thallus. (From Vogel, 1955.)...

Acarosporaceae Zahlbr. Thallus as given in the suborder diagnosis, little to distinct differentiation into medulla and cortex, the cortex usually with a distinctly developed paraplectenchyma. Apothecia solitary or several sunken in areoles, or sessile, with well-developed, more or less open or rarely punctiform disc. Spores ellipsoid to rarely globose, unicellular (but... 

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