TISSUES AND SIMPLE ORGANS. 53 



brings forth or awakens the activity of vegetative areas and ele- 

 mentary meristematic organs which through subsequent growth 

 and modification may subserve various functions. 



II. STRUCTURE AND FUNCTION OF THE EPIDEEMAL 

 TISSUE-SYSTEM. 



Living organisms, in general, are separated from their surround- 

 ings by a dermis or tegumentary tissue. The injurious influence of 

 its locnl or entire absence makes itself felt in various ways. Upon 

 examination we find that the epidermal system of plants has a three- 

 fold significance. 



1. The tegumentary tissue of plants, like the skin of animals, has 

 a mechanical function. The more delicate parts of plants require a 

 more resisting covering capable of protecting them against mechan- 

 ical injuries (pressure, friction, etc.). 



2. The dermis of land-plants forms a necessary protection 

 against evaporation in that it is highly impermeable to water and 

 water-vapor. 



3. In land-plants it also forms a water-supplying system. It 

 forms a peripheral enveloping water-storing structure as opposed to 

 the internal water-conducting tissue represented by the water-storing 

 cells and tracheal system (vessels, tracheids). 



These three functions correspond to suitable anatomical and 

 other adaptations. In each of the three groups of adaptations there 

 may be noticeable a slight development or a gradual increase up to 

 complete anatomical conformation, according to requirements. 



Epidermis may be defined as a superficial cell-covering of an 

 organ being at least one layer in thickness. If an increase in the 

 number of these cell-layers signifies an increase in the mechanical 

 or water-supplying function, the phytotomist speaks of it as a 

 " several"- or " many-layered " epidermis. Since these layers under 

 certain conditions frequently increase with great regularity, the old 

 expression cork- or periderm-formatiou has been used to designate 

 this change more specifically. Should this change still proceed in 

 a manner to be described later, it is designated as bark-formation. 



Before entering into the anatomical-physiological treatment of 

 the threefold tegumentary function it is important to note that in 

 the epidermis and vegetable teguments in general the cells are 

 closely united, not having intercellular spaces. This structural con- 



