A Tree is a Lit/ing Thing 



Sexual reproduction of trees plays 

 an important part in the development 

 of the diversity of our trees. By com- 

 bining characters of the pollen parent 

 with those of the seed parent, new com- 

 binations are formed, some of which 

 may be very valuable. But sexual re- 

 production is not absolutely necessary 

 for trees; many of them can be repro- 

 duced by vegetative means cuttings, 

 grafting, and budding. 



OLD AGE comes to trees, as to all 

 other living organisms. The span of 

 life of a tree is specific. Gray birch is 

 old at 40. The sugar maple lives longer, 

 up to 500 years. Some oaks may live 

 1,500 years, junipers 2,000 years. Some 

 of the giant sequoias are believed to 

 be about 4,000 years old. Old trees are 

 like old people the infirmities of age 

 are upon them. They have difficulty 

 with respiration (its rate in old plants 

 is much lower than in young plants) ; 

 the annual shoots are not so vigorous 

 as they once were, and the weakening 

 cambium activity is reflected in the 

 formation of fewer and fewer wood 

 cells. Hence, the annual rings become 

 narrower. As the rate of growth of 

 the tree decreases, dead branches ap- 

 pear in ever-increasing numbers. The 

 recuperative capacity of an old tree is 

 impaired, and its wounds do not heal 

 over so easily as before. The leaves be- 

 come smaller; their moisture content 

 decreases; the tree finds it more and 

 more difficult to provide water for its 

 vital functions; the inflow of food to 

 the growing points drops; and the 

 growth hormones probably cannot be 

 transported in large enough quantity to 

 the places where they are needed. 



Causes of death of a tree may be 

 numerous and are often difficult to 

 diagnose. When a tree is broken by 

 snowfall or uprooted by wind or killed 

 by fire, the cause of death is evident. 

 But often the cause is rather obscure. 

 Sometimes lack of water may cause 

 death of the tree, and again trees 

 weakened by drought may fall prey to 

 an insect or fungus attack. 



Fire is an archenemy of trees. Its 



direct effect on trees is obvious enough. 

 But there is also an indirect effect: 

 Heat may injure patches of succulent 

 inner bark of the tree trunk. Fermen- 

 tation may easily start in these places 

 and attract insects. The smoke of a 

 fire contains some physiologically ac- 

 tive gases ethylene, for example, or 

 acetylene. The gases may cause the 

 opening of the dormant buds prema- 

 turely, thus exposing them to frost 

 damage and contributing to the gen- 

 eral weakening of the tree. 



When a tree dies, its death almost 

 always can be traced to some external 

 cause cold, fire, drought, insects, 

 fungi, or malnutrition. Some of these 

 causes are beyond our control. Others 

 can be prevented. By taking good care 

 of the tree, one can prolong its life. The 

 tree should be well provided with 

 water and light and be well nourished, 

 or at least not deprived of nutritive 

 substances. A healthy tree will resist 

 attacks of insects and diseases; it will 

 develop a large crown and a strong 

 root system; and it will withstand the 

 action of the wind. 



If a tree is treated as a living organ- 

 ism, with an understanding of its vital 

 functions, it will be a constant source 

 of profit and pleasure to men. 



N. T. MIROV is plant physiologist of 

 the Institute of Forest Genetics, which 

 is a branch of the California Forest and 

 Range Experiment Station, maintained 

 by the Forest Service in cooperation 

 with the University of California, in 

 Berkeley. He holds a master's degree in 

 forestry and a doctor's degree in plant 

 physiology from the University of 

 California. 



The attention of the reader is di- 

 rected particularly to later chapters and 

 sections that relate to Dr. Mirov's sub- 

 ject, including the bibliography (For 

 Further Reference) at the end of the 

 booh; Pointers on Planting, by T. E. 

 Maki; First the Seed, Then the Tree, by 

 Paul O. Rudolf; Direct Seeding of 

 Trees, by W. E. McQuilkin; The Com- 

 munity of Trees, by Jesse H. Buell. 



