Prosoplectenchymatous tissues

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). 

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