52 BOTANY OF THE LIVING PLANT 



is best seen in tangential sections (Fig. 35). Such sections also show 

 the outline and relative position of the rays. Triangular intercellular 

 spaces may be seen where their cell-walls join : thus the ventilation- 

 system extends through them deep inwards into the vascular trunk. 



In temperate climates the activity of the cambium is interrupted 

 each winter. This leaves its mark on the woody column in the form 

 of annual rings, each of which represents the increment of growth 

 of one year. Consequently it is possible by counting them to estimate 



FIG. 38. 



Medullary ray of the Pine (em, tm, sm), seen in a radial section through the 

 cambial region (c) Phloem to the left, xylem to the right. ( x 240.) S. 



the age of the trunk. The reason why these rings are recognisable 

 is that the wood formed in the spring has larger and more numerous 

 vessels and on the average thinner walls than that formed as the 

 season advances. The physiological explanation of this is that the 

 first effort of the plant in spring is to increase the water-conducting 

 tissues, so as to supply the new leaves ; but in the later part of the 

 season, when nutrition is high, more is spent in thickening the walls, 

 thus adding to the mechanical strength. The result of continued 

 development of a woody trunk in the manner described is to give a 

 column constructed after the plan shown in Fig. 39, where the relation 

 of the annual rings and medullary rays is clearly shown. 



