xviii INTRODUCTION 



ties. Epidermal cells often exhibit the most conspicuous xeric 

 adaptations although these modifications commonly extend into 

 subepidermal layers, and even throughout the plant. 



The development of thick cuticles, infiltration of cutin and 

 waxes, and lignification of epidermal cell walls all represent devices 

 for laying a water barrier over the plant surface. Cutinized or 

 lignified hypodermal layers immediately beneath the epidermis are 

 also found among the xerophytes. In some xerophytes, sclerenchyma 

 with its hard, lignified walls is laid down to form insulating 

 sheets or fiber strands. 



Exceptional water resistance is found in cork. This tissue is 

 the product of a specialized form of cambial activity. The cork 

 cambium (phellogen) extends files of cells radially outward dis- 

 placing cortical tissues. Differentiation in these cells entails the 

 intercalation of lipoidal material (suberin) into the cell wall and 

 deposition of lignin upon it. After protoplastic degeneration cork 

 tissue consisting of water impermeable, air-filled cells remains. 



The presence of increased amounts of superficial lignin and 

 cutin not only provide water barriers, but may also serve as pro- 

 tective ultraviolet screens in regions of high light intensity. 



Under hydric conditions, the problems of insulation, support, 

 and radiation screening are minimized. Hence it is not surprising 

 to find that the epidermis in typical aquatic plants possesses ex- 

 tremely thin cuticles permeable to water and gases, and that stems 

 contain little or no lignin, even in vascular tissues, and no scleren- 

 chyma. 



The epidermal cell walls in seed coats sometimes resemble 

 those described in xerophytes. The preservation of dormancy 

 without desiccation for variable periods is often required in seeds. 

 Accordingly, seed adaptations must include adequate, but tempo- 

 rary insulation for the embryo. In addition to cutinization and 

 lignification, the seed coat exhibits a diversity of anatomical and 

 chemical modifications which implicate the cell wall. 



The few illustrations of wall and cell variations which have 

 been presented show clearly that cell and cell wall form, function 

 and constitution are indeed regulated by internal and environmental 

 factors alike. 



