Yearbook^ of Agriculture 1949 



somehow involved in the carbohydrate 

 translocation. It enters into the con- 

 struction of the cell wall; crystals of 

 calcium oxalate are found often in the 

 tissues of plants. Magnesium is a con- 

 stituent of the chlorophyll molecule. It 

 is also probably related to fat forma- 

 tion and to the synthesis of some pro- 

 teins. Potassium is especially abundant 

 in young growing parts of the tree; it 

 has something to do with synthesis and 

 translocation of sugars; in the absence 

 of potassium, cells do not divide. Iron 

 is needed to keep the tree green. Iron is 

 not a part of the chlorophyll molecule, 

 but without it chlorophyll cannot be 

 formed. Iron is also needed in respira- 

 tion. Generally, there is enough iron in 

 any soil, but sometimes in alkaline soils 

 it is found in an insoluble state. Iron- 

 deficient trees lack the healthy color. 

 The physiological role of minor ele- 

 ments is little known, but symptoms 

 of their deficiency are pronounced. At 

 present our concept of the physiology 

 of plant nutrition is in the process of 

 revision. With the recent advances of 

 nuclear physics, it is possible to prepare 

 radioactive mineral salts. "Tagged" 

 radioactive phosphorus or potassium 

 can be followed as soon as it is ab- 

 sorbed by a plant; it can be traced to 

 its destination and its function in plant 

 life can be determined. 



WATER is CONTAINED in all tissues 

 of a tree, both dead and alive. Young 

 leaves or tips of roots contain up to 

 90 percent of water; tree trunks con- 

 tain as much as 50 percent. Water is 

 indispensable to the tree. All living 

 processes take place in water. Sugars 

 are built from carbon dioxide and 

 water. Mineral nutrients are carried 

 from the soil to the top of the tree 

 in a stream of water. In the spring the 

 organic materials in the form of sugars 

 and amino acids are rushed in a stream 

 of water from their places of winter 

 storage to the bursting buds. 



And there is the dramatic process 

 called transpiration. In that process, 

 water is absorbed by the roots, pushed 

 into the sapwood, and then pulled up 



to the leaves (as high as 350 feet in 

 redwood) above the ground. The en- 

 ergy needed for transpiration, as for 

 photosynthesis, is supplied by the sun. 

 About one-half of the solar energy 

 falling on a leaf is used for transpira- 

 tion. Through the same openings (the 

 stomata) that admit carbon dioxide to 

 the inner tissues of the leaf, the water 

 is evaporated to the atmosphere, and 

 this evaporation creates a tremendous 

 pull on the minute, continuous strands 

 of water in the sapwood and thereby 

 causes a movement of water from the 

 roots to the treetop. There is no such 

 process in the tree as circulation of 

 the sap similar to circulation of the 

 blood in animals. Only a trifle of water 

 is transported from the crown down- 

 ward and comparatively little is re- 

 tained by the tissues. The terms "the 

 sap is up" and "the sap is down" are 

 not correct and are misleading. 



The formation of 100 grams of cellu- 

 lose requires 55 grams of water. But 

 while a tree increases its weight by 100 

 grams, it loses in transpiration nearly 

 100,000 grams (that is, 1,000 times 

 more) of water. 



Transpiration brings water from the 

 soil to the leaves so that photosynthesis 

 can be carried on. To enter through the 

 cell walls, carbon dioxide must be dis- 

 solved in water. The surface of the 

 chlorophyll containing cells must be 

 moist at all times. 



The leaves have a water-regulated 

 mechanism that permits a tree to shut 

 off the stomata and thus prevent loss 

 of water. But the very same stomata 

 have to be open in order to admit 

 carbon dioxide for the photosynthesis. 

 When stomata are open, the tree loses 

 water; when they are closed, the tree 

 cannot assimilate carbon dioxide. A 

 balance between the two processes 

 must be maintained by the tree. 



The stomata open their little shutters 

 early in the morning. At noon they 

 begin to close, and just before sunset 

 they are closed tight for the night. In 

 some trees, stomata may open at night. 

 During excessively hot and dry days 

 the stomata are open only for a short 



