262 A TEXTBOOK OF BOTANY [Ch. V, 10 



ologically-determined tissue has become fixed as a distinct 

 morphological tissue system, called the dermal system 

 (Fig. 182). 



2. Photosynthesis, or formation of food under action 

 of light. This function requires a superficial position, but 

 protection by the epidermis, and hence comes immediately 

 beneath the latter, which is transparent. Tissue, chloren- 

 chyma, which covers young stems, and extends out into flat 

 projections, the leaves; in older stems ceases to be green, 

 merges into bark, develops cork cambium and layers of 

 cork in replacement of the vanishing epidermis, and grows 

 to allow for expansion of the stem ; in roots it merges over 

 into the cortical layer. Its innermost layer forms a special 

 starch-sheath in young stems, or endodermis in young 

 roots, having perhaps a perceptive function. This tissue 

 has become fixed as a distinct morphological system called 

 the cortex (Fig. 182) or cortical system. 



3. Absorption of water, mineral matters, and carbon 

 dioxide. This function requires an external position and 

 freedom from interference of the epidermis. Tissue, for 

 water and mineral matters, an external hair-layer on the 

 roots, continuous with the epidermis of stem and leaves, and 

 a thick cortical layer, continuous with the chlorenchyma. 

 The absorption of carbon dioxide by the cortex involves 

 no special tissues, but depends on stomatal breaks in the 

 epidermis, and intercellular air spaces, which are especially 

 large in chlorenchyma. 



4. Transpiration, incidental to gas absorption, involves 

 no tissues, but the need for its control explains the presence 

 of the regulating guard cells of the epidermis. 



5. Conduction of water and food through the plant. It 

 requires a position in contact with the water-absorbing 

 and food-making cortex, and an elongated tubular con- 

 struction. Tissues, xylem, comprising ducts and asso- 

 ciated elements : phloem, comprising sieve tubes and as- 

 sociated elements : in leaves the bundle sheath, — the 



