DERMAL SYSTEM IN LICHENS 



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Dxrmr 



walls are also frequently wavy. Ue Bary long ago remarked upon 

 the close resemblance which exists between such a superficial layer and 

 the thick astomatic epidermis of a Vascular Plant. 



It is among Lichens, which so often grow in situations where they 

 are exposed to sudden fluctuations of the climatic conditions, that an 

 elaborately constructed and correspondingly efficient dermal tissue 

 most frequently occurs. In Usnea barbata and other fruticose Lichens 

 the dermal tissue consists of a fairly thick layer of closely interwoven 

 hyphae, the walls of which are so greatly thickened as almost to 

 obliterate their cavities. In certain foliose Lichens, again (Parmelia, 

 Physcia, Sticfa, Peltigera, etc.), it is composed of isodiametric cells with 

 more or less thickened walls, arranged in several perfectly continuous 

 layers. The nearest approach, however, 

 to the condition of a typical epidermis 

 is found in certain gelatinous Lichens 

 ( Leptogium, Obryzum, Mallotiu m ). Here 

 the dermal tissue usually consists of a 

 single layer of polyhedral or tabular 

 cells, which are in uninterrupted con- 

 tact with one another : the outer walls 

 of these cells are often somewhat 

 thickened. In Mallotium Hilclebrandii, 

 which has been examined in some 

 detail by the author, the superficial 

 tissue, on the upper side of the thallus, consists of a single layer of 

 isodiametric cells with thin, colourless lateral and inner walls and 

 greatly thickened outer walls. The outer wall is further differ- 

 entiated into two sharply contrasted layers. The outer and thicker 

 of these is colourless and homogeneous; it is bounded towards the 

 outside by a relatively dense pellicle, which, however, in no way 

 corresponds to a cuticle. The inner stratum is thin and reddish- 

 brown in colour : like the cellulose layer of a typical outer 

 epidermal wall it is continuous with the substance of the lateral 

 walls. Treatment with concentrated sulphuric acid leads to the 

 complete solution of the thick outer layer (which in this respect 

 agrees with the walls of the medullary hyphae), while the thin, brown 

 inner layer and the lateral and inner walls are not dissolved, and do 

 not even swell up. Whereas, therefore, in the ordinary epidermis of 

 a Higher Plant, it is the outermost layer of the outer wall which is 

 chemically specialised (cutinised), the conditions are, in this respect, 

 exactly reversed in Mallotium. Whether it is permissible to regard 

 the innermost layer as cutinised, in this case, is another matter. The 

 thickened outer wall is certainly quite pervious to water; for if .) 



Fig. 45. 



Upper surface of the thallus of Mallotium 

 Hildebrandii, in vertical section, showing the 

 epidermal layer. 



