DERMAL SYSTEM IN ALGAE AND FUNGI 



145 



V. THE DERMAL SYSTEM AMONG THALLOPHYTA. 



In those Algae which lead a submerged existence, the superficial 

 layer of the plant-hody will obviously possess only those features of a 

 typical epidermis that are connected with its mechanical function or 

 with its action as a light-screen. The mechanical factor finds expression 

 in the presence of thickened outer walls and in the development of 

 numerous vertical partitions in the superficial layer, which increase its 

 power of resisting radial pressure. A typical epidermis is scarcely ever 

 present, chiefly because submerged plants have no need to guard against 

 the risk of drying up, so that the most important of the external 

 agencies which affect the development of an epidermal layer is absent, 

 but also because, in these circumstances, no reason exists why the 

 photosynthetic system should not extend its 

 sway over the outermost layer of the plant-body. 



In certain Sea-weeds, especially among the 

 Ehodophyceae (spp. of Chylocladia), peculiar 

 iridescent plates are found in the superficial 

 cells ; these normally occupy the outer walls, but 

 shift on to the lateral walls when illumination 

 is feeble. On account of their behaviour, 

 Berthold 85 believes that these bodies act as 

 screens or reflectors, which intercept part of the 

 light that falls upon the plant. It must be left 

 undecided whether this is the correct inter- 

 pretation of their significance or not. 



According to Berthold, the hair-like appendages which occur in 

 various Sea-weeds also serve for protection against excessive illumina- 

 tion. This view is supported by a variety of facts. In the case of 

 many species which have a tufted habit, hairs are entirely absent from 

 the branches that are hidden in the interior of the tufts ; other species, 

 again, are devoid of hairs in winter-time, or where the water is some- 

 what turbid, or when they grow in the shade of rocks, but are densely 

 hairy if they are brightly illuminated. It is probable, however, that the 

 hairs of Algae are in many instances concerned with a totally different 

 function, namely, the absorption of food- materials. 



Among Fungi the degree of specialisation of the dermal tissue varies 

 according to the special functions, and more especially according to the 

 longevity, of the organ concerned. The short-lived fleshy or juicy 

 sporophores which are characteristic of so many Basidiomycetes may be 

 compared with the fugacious floral structures of Phanerogams, as 

 regards the development of their dermal system ; the relatively moderate 

 demands which are made upon this system in their case are satisfied by 



Fig. 42. 



L.S. through the superficial 

 region of a pileus of Polyporus 

 lucidus. xl90. After DeBary. 



