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. 
