178 THE GROWTH OF BOOTS AND STEMS. 



down by the weight of the leaves, and the winter-position, when it has 

 risen owing to the loss of the leaves, remains practically the same year 

 after year. It is most important to bear in mind the principle of 

 correlation of growth, one aspect of which is beautifully illustrated 

 by the cambium in connexion with secondary thickening. In examin- 

 ing felled trees in a plantation, or lopped branches, note the frequently 

 excentric appearance of the wood-layers (annual rings) and try to 

 account for it in each case. 



208. Annual Rings of Wood. In a cross section of a 

 tree trunk or a thick branch, 1 a series of concentric rings 

 (really, of course, layers) can be seen in the wood. Each of 

 these rings corresponds (normally) to one year's growth. 



The wood formed in spring has large vessels which are 

 required for the rapid transport of water to the young leaves, 

 and it is softer than the hard compact wood formed later in 

 the year as the rapidity of growth decreases and the leaves 

 transpire and assimilate less and less actively. In autumn 

 the formation of new wood ceases, reserve foods are stored 

 up, and the resting-buds (Ch. VI.) are usually by this time 

 fully developed. 



In spring the buds open and the young leaves expand ; the 

 cambium, being well supplied with food, again begins to grow 

 actively and to form new spring wood. The transition from 

 spring to autumn wood may be gradual, but there is an 

 abrupt change from the autumn wood (containing narrow 

 vessels) of one year to the spring wood (containing wide 

 vessels) of the next. 



209. Rays. Running outwardly from the centre of the 

 stem are numerous bands, the medullary rays, 2 which serve 

 partly for the storage of starch and oil, and partly for the 

 horizontal conveyance of food materials and water. They 

 also serve to bind together the successive layers ("rings ") of 

 wood, and thus make the wood compact and strong. The 

 rays are vertical plates ; their height varies, but they must 

 not be thought of as sheets of tissue running continuously 



1 Old roots undergo growth in thickness (beginning in a rather 

 different way), just as stems do, and show annual rings. 



2 The rays, though called medullary or pith rays, do not all extend 

 into the pith ; most of them stretch from the cambium to an annual 

 ring (additional rays are formed each year). 



