8 



Yearboo\ of Agriculture 1949 



seasonal growth; it prepares for the 

 winter. Evergreen trees retain their 

 foliage for winter, but deciduous trees 

 act differently. They remove much of 

 the nutrient material from the leaves; 

 then a peculiar physiological process 

 (abscission) of the leafstalks causes the 

 leaves to drop. The tree is in a deep 

 rest now and can withstand a great 

 deal of cold. An unusual warm spell in 

 late winter or early spring, however, 

 may cause buds to open subsequent 

 cold may kill them. Sometimes a north- 

 ern tree transplanted in the South 

 might open its buds too early in the 

 spring and suffer from a later frost. 



REPRODUCTION is possible when the 

 tree reaches its physical maturity. 

 The sexual reproduction of trees is 

 basically similar to that of animals. In 

 plants, reproduction manifests itself by 

 the appearance of male and female 

 flowers, which may be borne either on 

 separate trees, as in cottonwood, or on 

 the same tree, as in pine, or even com- 

 bined into a perfect flower, as in the 

 magnolia. 



Pollen of the male flower fertilizes 

 the ovule of a female flower, which 

 then develops into the seed. The whole 

 process of reproduction involves set- 

 ting of flower buds, development of 

 the male and female flowers, pollina- 

 tion, and development of the seed and 

 fruit. Each process depends on internal 

 as well as external conditions. 



In annual plants, the reproductive 

 stage means subsequent death; the 

 plant dies as soon as the seed is ma- 

 tured. In trees, production of seed is 

 continued for many years. In a tree 

 there is a delicate balance between 

 vegetative growth and reproduction. 

 If a tree grows too fast, it will not pro- 

 duce much fruit or seed. The repro- 

 ductive stage is generally reached when 

 a tree begins to slow down its most 

 vigorous height growth. 



The accumulation of carbohydrates 

 is conducive to the flowering, while the 

 abundance of the minerals, especially 

 nitrogen, promotes growth at the ex- 

 pense of reproduction. The proper bal- 



ance between organic and mineral nu- 

 trition and the possible formation of 

 flowering hormones occurs in the tree 

 only after a certain stage of maturity 

 has been reached. After that, a tree 

 begins to produce seed, but not neces- 

 sarily every year. Seed bearing is a 

 taxing process. Much material and 

 much energy are required for it. Many 

 trees have periodicity of seed years, 

 and the intervals between the good 

 seed years vary in the different trees. 

 Again, this periodicity apparently de- 

 pends on a definite combination of 

 nutritional and external factors. As the 

 flower buds are set during the previous 

 summer, the weather conditions of the 

 last year have a lot to do with the 

 flowering. Dry, warm weather gener- 

 ally is favorable for setting flower buds. 

 Weather conditions prevailing during 

 pollination and the development of 

 pollen grains are also of importance. A 

 great deal of light is needed for flower- 

 ing. Trees grown in the open produce 

 flowers and seed in profusion and much 

 earlier than trees that are grown in the 

 shade. Abundant sunshine at the time 

 of setting flowering buds also con- 

 tributes to the seed crop the next year. 



The effect of photoperiod, or day 

 length, discovered in 1920 by W. W. 

 Garner and H. A. Allard of the United 

 States Department of Agriculture, is of 

 great importance in flowering of trees. 

 Some trees flower only when the days 

 are short, while others bloom when the 

 days are long. A northern tree that was 

 growing under the long-day conditions 

 may not bloom if moved to the South, 

 because the summer days of the new 

 home are too short for it. If, say, a street 

 lamp provides that extra needed light, 

 a northern tree may burst into bloom 

 even in a short-day country. 



Certain chemical substances, such 

 as ethylene, are known to break the 

 dormancy of plants. If your lilac bush 

 unexpectedly bursts into bloom earlier 

 than usual, it might be because you 

 had burned some fallen leaves or clip- 

 pings nearby and the smoke supplied 

 enough ethylene to awaken the dor- 

 mant flower buds. 



