286 LEAVES 



Stored. in all forms of food there is latent energy, which is trans- 

 formed sunlight, and through respiration the foods are broken 

 into simpler compounds with the release of energy, which is 

 utilized in the various kinds of work of the plant. 



Summary 



Leaves may be classed as primary and secondary. The 

 primary leaves, represented by the cotyledons, are mainly storage 

 organs and are usually short-lived. The secondary leaves form 

 the foliage of plants and are the food-making organs. The form 

 and arrangement of leaves vary much, but usually result in the 

 best exposure to light. 



The chief tissues of the leaf are the epidermis consisting of pro- 

 tective cells and stomata, the mesophyll containing the working 

 cells, and the veins, which give strength and supply other tissues 

 with water and salts and carry away the manufactured products. 



The processes taking place in leaves are photosynthesis-, trans- 

 piration, and respiration. Leaves are especially adapted to 

 photosynthesis because of their green tissue and exposure to 

 sunlight. Upon photosynthesis the carbohydrate supply of the 

 world depends. Respiration is not peculiar to leaves; for it 

 takes place in all living cells, but can be easily observed in leaves. 

 Leaves are much exposed to transpiration, which may benefit 

 the plant or result in injury. The amount of water lost through 

 transpiration depends upon the character of the transpiring 

 surface, temperature, humidity of the air, light, and velocity of 

 wind. A plant may be protected against the dangers of trans- 

 piration by having its transpiring surface modified or by being 

 able to supply water from the soil or storage organs in sufficient 

 quantities to meet the loss through transpiration. 



Leaves may have become modified into special forms, such as 

 scales, tendrils, thorns, pitchers, or traps. 



Through metabolism the photosynthetic sugar is chemically 

 combined with mineral elements to form proteins and proto- 

 plasm, transformed into materials which constitute a frame- 

 work, changed into storage foods and into various other kinds 

 of plant products. Through respiration, a phase of metabolism, 

 plants obtain chemical energy by releasing the latent energy of 

 sugar or of the compounds of which it is a part. 



