4 U. S. DEPARTMENT OF AGRICULTURE. 



vapor from the moist intercellular spaces escapes into the 

 air, and carbon dioxide and oxygen exchange between the 

 cells and the air takes place. The loss of water by the living 

 plant through evaporation is known as transpiration. 



When unbroken, the epidermis prevents excessive loss of 

 water from the moist tissues underneath and keeps out for- 

 eign organisms such as fungi and bacteria. In older stems 

 and roots, such as the Irish potato, the protective covering is 

 composed of cork layers, in which the breathing pores are 

 present as lenticels. 



The supporting tissues, such as the veins of leaves or the 

 fibers of stems, are composed of woody or thickened cells. 

 The conducting tissues, found in the veins of leaves or the 

 ring tissues of stems or roots, translocate the water and min- 

 erals taken in by the absorptive tissues of the roots, and the 

 food made in the food-making tissues. 



The green color of vegetation is due to a substance known 

 as leafgreen or chlorophyll. By virtue of chlorophyll, and 

 with the aid of sunlight, the green tissues of plants are able 

 to manufacture carbohydrates (sugars and starches) from 

 the carbon dioxide of the air and the water of the soil. 



These carbohydrates are the basic substances from which 

 the other principal food of plants and animals, such as fats, 

 oils, and proteins, are manufactured by green and non-green 

 cells. 



Many plants possess tissues in which they accumulate 

 excess food materials. These are the storage tissues. The 

 Irish potato and the sweet potato are to a large degree com- 

 posed of storage cells in which starch and sugar, respec- 

 tively, are stored. 



Digestion and Respiration. 



The stored food of the potato is in the form of starch 

 grains. In the spring when the potato sprouts, the insoluble 

 starch is changed to soluble sugar, to which is due the sweet 

 taste of potatoes at that season. In ripening plant tissues, 

 insoluble, unpalatable substances, such as starch and tan- 

 nins, become soluble, aromatic, and palatable. 



Another process which goes on in all protoplasm, plant or 

 animal, is respiration. The making of food out of simple 

 materials is a constructive process, whereas respiration is 

 destructive. In its simplest form, it involves a consumption 

 of oxygen, a burning or oxidation of the substances of the 

 cell, such as sugar, starch, fat, and proteins, and a liberation 

 of carbon dioxide gas, water, and energy. Part of this 

 energy is manifested as heat. The so-called "heating" of 

 crops in transit and storage generally is due to the heat 

 evolved in respiration. 



The rate of respiration is greatly influenced by tempera- 

 ture. At high temperatures, respiration proceeds at a rapid 

 rate, and the food substances are soon burned up. At low 

 temperatures, the rate of respiration is so retarded that it 



