A Tree is a Uving Thing 



time in the early morning and then 

 close for the rest of the day. Under 

 these conditions the tree cannot make 

 much sugar from the carbon dioxide. 

 What can a man do to help a tree 

 in its water economy? Not more than 

 to supply it with water by irrigation, 

 by preserving the natural mulch on the 

 ground and thus reducing evaporation 

 from the soil, and by not planting trees 

 too close to each other or exposing the 

 shade-loving trees to full sunlight. 



THE PHYSIOLOGY OF GROWTH is 



this: Through the process of photo- 

 synthesis and with the help of nitro- 

 gen and the mineral elements, the tree 

 builds up its body. In some trees, such 

 as the giant sequoias, as much as 50,- 

 000 cubic feet of organic matter, 

 mostly wood, may accumulate in this 

 way. But the growth of a tree is not 

 merely an accumulation of organic 

 matter. Growth is an involved physio- 

 logical process, in which the use of 

 building materials is regulated by the 

 growth substances or hormones. 



Growth of a tree is retarded if min- 

 eral nutrition is held at a minimum 

 and water is withheld. This is the meth- 

 od used by the Japanese in dwarfing 

 trees ; some of their dwarf trees, grown 

 in small pots, may be several hundred 

 years old. In fact, any pot-bound young 

 tree is checked in its growth and thus 

 is more or less dwarfed. 



Growth of the tree depends not only 

 on the correlation of its physiological 

 functions but also on external factors 

 like temperature, light, and moisture. 

 Within a certain range, an increase of 

 temperature of 18 F. nearly doubles 

 the rate of growth of plants ; but when 

 temperatures are either too low or too 

 high for a proper functioning of the or- 

 ganism, many disturbances may occur 

 in the tree. In that case the growth of 

 the tree is retarded, and although life 

 may still continue there is no coordina- 

 tion between the different vital func- 

 tions. The tree ceases to grow. The 

 optimum temperature for growth is not 

 necessarily the same as the optimum 

 temperature for general development 



of the plant. Many trees need a low 

 temperature period for their normal 

 development; when this cold period is 

 eliminated, they do not grow. 



Light must also be available in the 

 proper amount and quality. When light 

 is lacking, the tree cannot manufac- 

 ture organic matter and will eventually 

 die. Light also retards the growth of the 

 tree. In the dark, the shoots grow 

 faster than in the light. In yellow and 

 red light, the plant can assimilate car- 

 bon dioxide very well, but the plant 

 does not develop normally it behaves 

 as if it were growing in the dark. For 

 normal development a tree needs, be- 

 sides yellow and red light, the blue, 

 violet, and ultraviolet rays of the sun. 

 Those rays are not needed for photo- 

 synthesis, and their action on the 

 growth is that of retardation. The blue 

 end of the spectrum is needed by a 

 tree for formative purposes. 



When a tree is bent by some me- 

 chanical force, such as the wind, its 

 normal growth is disturbed. On the up- 

 ward side of the tree, the newly formed 

 cells of the sapwood are stretched; on 

 the lower side, they are compressed. 

 This distortion of the wood structure, 

 due to pressure, is often noticed in our 

 conifers. Where a pressure is applied, 

 there is formed so-called compression 

 wood, which lowers the quality of lum- 

 ber manufactured from such wood. 



In a temperate climate, trees show 

 an annual periodicity of growth. The 

 annual shoot completes its growth early 

 in the season, say at the end of June. 

 By that time, in many forest trees, all 

 cell division for the next year's growth 

 is completed in the bud. This means 

 that the next year's growth pattern of 

 a tree is determined almost a year be- 

 fore all microscopic flower buds are 

 set; all microscopic leaf buds are 

 formed. The next season the growth 

 takes place mostly by elongation of 

 the bud cells prefabricated in the pre- 

 vious year. Growth in diameter takes 

 place throughout the summer by divi- 

 sion and enlargement of cambium cells. 



A long time before cold weather sets 

 in, the tree has already completed its 



