CHAPTER III. 



THE DERMAL SYSTEM. 



/. GENERAL CONSIDERATIONS. 



Even in the case of the simplest of unicellular Algae, the living sub- 

 stance has found it necessary to shut out the surrounding medium 

 and thus to withdraw from the immediate influence of hostile environ- 

 mental conditions by secreting a protective membrane or cell-wall. 

 Highly organised multicellular plants stand in still greater need of a 

 tegumentary or dermal system which will serve to protect their internal 

 tissues. Among the Lower Plants the cell-wall serves both as a limit- 

 ing membrane and as an exoskeleton. In the case of Higher Plants, 

 on the contrary, the dermal and the mechanical functions are assigned 

 to distinct tissue-systems in accordance with the principle of division 

 of labour. 



The physiological anatomy of the dermal system will be more 

 readily comprehended after some account has been given of the 

 principal agencies against which this system is intended to afford 

 protection ; in this way it will at once become evident what conditions 

 a tissue must fulfil, in order to rank as part of the dermal system. 



The discussion may at first be confined to the aerial organs of 

 ordinary terrestrial plants. Since such organs are liable like other 

 freely exposed moist bodies to lose water by evaporation, the chief 

 danger by which they are threatened is that of desiccation. Next in 

 importance is the risk of mechanical injury, such as may be caused by 

 the impact of violent rain, blown sand or dust, by the attacks of snails, 

 insects or other animals, and so forth. Certain tissues also need pro- 

 tection against excessive illumination or against loss of heat by 

 nocturnal radiation. During winter, finally, the living cells in perennial 

 twigs and branches and in other hibernating organs have to be preserved 

 from violent fluctuations of temperature and especially from the effects 

 of sudden thawing. 



