102 PLANT PHYSIOLOGY AND ECOLOGY 



during the process of respiration. The latter is aerobic when it 

 takes place in the presence of oxygen, and anaerobic when oxygen 

 is absent. Aerobic respiration is characteristic of green plants, 

 though these are to a certain extent anaerobic during germina- 

 tion. Anaerobic respiration occurs chiefly in fungi. During 

 respiration in flowering plants, oxygen is taken up from the air, 

 and carbon dioxide is evolved, accompanied by the production 

 of heat. The process is one of oxidation, in which the living sub- 

 stance or the elaborated foods in it serve as fuel for the produc- 

 tion of energy. It occurs in every living cell, but is most pro- 

 nounced in regions of greatest activity, especially so in meristem. 

 It is at a minimum in resting cells, and is practically absent in 

 seeds and other propagative organs during the low temperatures 

 of winter. Respiration is directly dependent upon temperature, 

 but is little affected by light. The minimum temperature for the 

 process is about L5 C. Its activity increases with the tempera- 

 ture, and appears to reach an optimum in the neighborhood of 

 the maximum for flowering plants. 



124. Fermentation. Bacteria, yeasts, and molds obtain the 

 necessary supply of energy by decomposing the greater part of 

 the food upon which they grow into alcohol or various organic 

 acids. This process, which is called fermentation, may be carried 

 on by both aerobic and anaerobic plants, and is merely a kind of 

 vigorous respiration. The power to carry on fermentation is 

 small or absent in many fungi. It is especially characteristic 

 of yeasts, and the consequent action is well illustrated by the 

 common yeast. The latter decomposes sugar into alcohol and 

 organic acids, with the evolution of carbon dioxide. Yeast fer- 

 ments sugar and other carbohydrates normally in the presence of 

 oxygen, but it may manifest this activity for some time without 

 oxygen. Yeasts may act upon fats, but not upon proteids, while 

 bacteria and molds produce fermentation in sugars, oils, or pro- 

 teids. 



125. Germination. Seeds owe their ability to germinate under 

 the proper conditions to the energy stored in the reserve food 

 contained in them. This energy is released by the processes of 

 digestion and respiration. The reserve food of seeds is usually 

 in the form of starch, oil, or proteid, and in many seeds two of 

 these occur together. It is either stored in the cotyledons or 

 packed about them as endosperm. In both cases its digestion 



