CHAPTER XX 

 THE WOODY STEM 



General Characters of Woody Plants. — We saw in 



Chapter XV that perennial herbaceous plants maintain 

 their vegetative bodies from year to y§ar by means of a 

 persistent underground shoot (rhizome, tuber, corm or 

 bulb), which throws up new aerial shoots at the beginning 

 of the next growing season. New portions of the rhizome 

 are formed every year, new tubers, corms or bulbs are 

 produced, the older parts of the rhizome, the old tubers, 

 corms and bulbs dying off. The aerial shoots of woody 

 plants, on the other hand (trees and shrubs), do not die 

 down every year, leaving only the rhizome or other 

 form of underground shoot to continue growth the 

 next year, but themselves continue growing from year 

 to year, the terminal (or lateral) buds of the branches 

 forming a fresh portion of shoot in each growing season, 

 this being a direct (or indirect) continuation of the portion 

 formed the year before. At the same time the portions 

 of stem formed in earlier years grow in thickness each 

 year, and are thus able both to support and to conduct 

 water and salts from the root to the constantly increasing 

 shoot system above. This process of growth in thickness 

 is called secondary thickening, and is brought about by 

 the activity of the cambium or secondary (vascular) 

 meristem, whose beginnings we have already noted. 

 In most climates, also, there is a definite season of the 

 year unfavourable or impossible for growth (either a 



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