S6 PLANT LIFE. 



temperate regions there is one period of active growth, 

 consequently one portion is added by the cambium yearly 

 to the xylem, and as these yearly additions of wood are 

 usually clearly distinguishable they are called annual rings ; 

 in the trunks of some tropical dicotyledonous trees more 

 than one ring is produced annually. The concentric rings 

 observable in the trunks of our forest trees when sawn 

 across correspond to annual rings of wood or xylem, and 

 are produced as follows. During the spring, when the 

 daughter-cells of the cambium first become transformed 

 into xylem elements, the pressure of the surrounding cortex 

 and bark is comparatively slight, consequently the cells 

 of the newly-formed wood are large and thin walled, and 

 consist to a considerable extent of large vessels. As the 

 season advances the pressure of cortex and bark becomes 

 much greater, consequently the new wood is more com- 

 pressed, resulting in the cells having smaller cavities and 

 thicker walls, and vessels are almost or entirely absent ; 

 hence the wood becomes gradually denser during its annual 

 development. Next season the spongy spring wood com- 

 mences abruptly upon the dense wood of the previous 

 autumn, thus sharply differentiating each annual ring. In 

 Coniferm, an order including pines and yews, in the secondary 

 wood, that is, all wood formed after the first year, vessels 

 are entirely absent, the wood consisting entirely of wood- 

 fibres with bordered pits. In this case the distinction of 

 annual rings depends entirely on the relative size of the 

 cells, and the thickness of their walls. The primary wood 

 of Conifers, that is, the wood produced during the first 

 year's growth, contains vessels. 



In roots the fibro-vascular bundles are usually fewer in 

 number than in the stem, and are also differently arranged ; 



