SYSTEMS OF TISSUES AND PLANT GROWTH. 165 



417. During the season of activity the young stem 

 continues to increase both in height and diameter by 

 the multiplication of cells and the formation of the 

 various tissues required by the conditions of growth 

 (Chap. III., Introduction) ; hence a mass of infant cells 

 is constantly present between the wood and the bark, 

 and in the buds of the stem and branches. 



On the approach of winter the leaves fall, the ter- 

 minal buds refuse to expand, and the entire process 

 of growth is arrested, until the revivifying warmth of 

 the succeeding spring unlocks the imprisoned forces 

 that have slept during the frosty season, when the 

 fluids from the earth begin to flow upward and out- 

 ward through the vessels and ducts of the last year's 

 wood to the bark and the leaves ; the young cambium 

 cells which have slumbered through the winter are 

 filled with sap and commence another season of 

 growth ; the buds burst into leaves or flowers, and the 

 greatest activity succeeds the late period of rest. The 

 young cells multiply and increase in size, most of 

 them being changed into woody fiber and ducts, com- 

 mencing a new layer of wood on 

 the outside of the last layer, and 

 a new layer of bast on the inside 

 of the old one ; also a new layer 

 on the inside of the cortex layer. 

 In this way the work goes on, and 

 layer after layer is added for each 

 period of activity, which in regions ^^jjjjjj^?' 517 

 of severe frost occurs yearly (Fig. 517, is a photograph of & cross- 



T-rT' T section of an oak-stem twenty-five 



517). Within the trOpiCS and all years old, showing the annual circular 



rings, the whole surrounded by the 



regions of no frost, periods of rest rough bark. 



and activity may occur more frequently than once a 



