Yearbook^ of Agriculture 1949 



temperature) dextrose sugar; although 

 some energy is lost as heat, most of the 

 energy released during the process is 

 used by the organism for its vital proc- 

 esses. Thus sugar is a source of energy 

 for a tree just as it is for a football 

 player. The chemical reaction of res- 

 piration is a reversal of the chemical 

 reaction of photosynthesis, as seen from 

 the following scheme: 



PHOTOSYNTHESIS: Carbon dioxide 

 + water + energy expended ^dextrose 

 + oxygen; 



RESPIRATION : Dextrose + oxygen-* 

 carbon dioxide + water + energy re- 

 leased. 



In daytime both photosynthesis and 

 respiration occur at the same time. 



Oxygen liberated in photosynthesis is 

 used for respiration, while the carbon 

 dioxide exhaled by the tree is used in 

 photosynthesis. As photosynthesis is a 

 more intensive process than respiration, 

 during a normal day an excess of oxy- 

 gen is eliminated and an excess of 

 carbon dioxide is absorbed by the tree. 

 When, under adverse conditions, day- 

 time respiration is more intensive than 

 the body-building photosynthesis, the 

 tree loses weight instead of gaining. At 

 night, because of the absence of light, 

 photosynthesis is at a standstill, but 

 respiration continues just as in hu- 

 mans, oxygen is taken in and carbon 

 dioxide is eliminated. Respiration is 

 going on at all times in all living cells, 

 in the leaves, the roots, and in the stem 

 and bark. 



While photosynthesis has its opti- 

 mum in cool days and decreases when 

 the weather becomes too hot, respira- 

 tion does not have such an optimum. 

 The warmer it gets, the more intense 

 is the respiration. Respiration is less 

 sensitive to the lack of water than 

 photosynthesis; that is why during 

 droughts, when photosynthesis stops, 

 respiration still continues and causes 

 great harm to the tree. Inside temper- 

 atures of 120 to 130 F. are deadly. 



NITROGEN is needed by a tree for 

 making its proteins. Without proteins 

 a cell cannot grow and cannot divide. 



Generally speaking, an abundance of 

 nitrogen promotes vegetative growth 

 of a tree. Animals have no capacity 

 for producing proteins from nitrogen ; 

 they depend on plants for the needed 

 proteins. 



A tree has the capacity to absorb 

 inorganic nitrogen and with it to make 

 its own proteins. Although four-fifths 

 of the air consists of nitrogen, less than 

 1 percent of the element is found in the 

 wood of a tree. And to get that little 

 bit of nitrogen is an extremely diffi- 

 cult task for a tree. Nitrogen as found 

 in the atmosphere cannot be used by 

 the tree; it has to be converted into 

 ammonia or into nitrates and only in 

 this form (mostly as nitrates) can ni- 

 trogen be absorbed by the roots. Let 

 us see how a tree manages its nitrogen 

 economy. 



Traces of ammonia are found in 

 the air, and some of the nitrogen oxide 

 is formed there, especially after thun- 

 derstorms. These substances are car- 

 ried by the rain to the soil, but their 

 quantity is altogether too meager to 

 contribute much to the nitrogen nutri- 

 tion. A few trees, such as the locust or 

 alder, have on their roots nodules 

 formed by bacteria that are capable of 

 assimilating nitrogen from the air and 

 converting it into nitrates, but most 

 trees have no nitrogen-fixing nodules. 

 There are free bacteria that live in the 

 soil and can use atmospheric nitrogen. 

 But these bacteria are not abundant 

 and they like warmth, so that in cooler 

 climates they are not active. Fallen 

 leaves, if not burned, contain some 

 proteins. These proteins are gradually 

 decomposed into amino acids, ammo- 

 nia, and eventually into nitrates. But 

 fallen leaves contain only about 1 per- 

 cent of nitrogen slightly more in the 

 hardwood leaves and slightly less in 

 pine needles. 



Animals waste a great deal of ni- 

 trogen, which they obtain from the 

 plants. Fur, hair, nails, and skin, be- 

 ing made of proteins, contain nitrogen 

 that cannot be used again by the or- 

 ganism. Large amounts of nitrogen are 

 eliminated by the animals as waste. 



