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TEXTBOOK OF BOTANY 



localities for centuries; but since a part of the underground roots, or 

 stems, dies each year and new parts are added each year, the age of 

 the oldest part of the plant body is rarely more than 3 to 10 years. 



Trees and shrubs are woody perennials; they may live 5 to 10 years 

 or longer before they begin to bear flowers, fruits, and seeds. Any horti- 

 culturist or forester will tell you that there is great variation in the 

 abundance of reproductive structures in trees from year to year and that 

 certain trees bear fruits and seeds only once in several years. This 

 periodicity is dependent in part on weather and soil conditions, and in 

 part on heredity. 



Winter aspect. The most distinctive feature of plant life in winter is 

 the donnancy of most plant organs (Fig. 21). The internal causes of 





Fig. 21. A forest in winter when all the aerial parts of plants are dormant. Photo 



by C. H. Jones. 



dormancy so characteristic of late summer and autumn usually disap- 

 pear during the winter months, but owing to low temperatures no growth 

 occurs. Many plants will start developing at this time if they are moved 

 into a greenhouse. 



Nevertheless, some processes continue within plants that have every 



