68 OAK. [CH. V 



seedling, rays of tissue passing between the bundles ; 

 these should still exist, and they can in fact be clearly 

 seen (together with other medullary rays of later origin) 

 running radially outwards, as shown in fig. 29. One 

 other point can be made out in the same way ; the wood 

 of the stem or branch increases in size every year by the 

 conversion of a number of cambium cells into woody 

 tissue, and since the cambium is in the form of a hollow 

 cylinder, giving a ring in section, it is clear that a ring 

 of wood must be added every year ; these are the con- 

 centric markings seen on the section of the stump, from 

 which the age of a felled tree can be calculated. These 

 circles, known as annual rings, are shown in the section 

 of an oak stem, in figs. 28 and 29. 



With a simple lens or a low power it can be seen why 

 the annual rings are so clearly marked out. The con- 

 centric circles visible to the naked eye are shown in 

 fig. 30 to consist of lines of large vessels. If the eye 

 travels from the centre to the circumference of the section, 

 it will be seen that it is the central margin of each annual 

 increment that is marked by a line of large vessels. In 

 each annual layer the vessels become smaller and less 

 frequent at the peripheral margin, till at the beginning 

 of the next year's growth the row of large vessels again 

 suddenly appears. But even in places where the vessels 

 are absent the ring can be detected by the close texture 

 of the non-vascular elements formed in autumn. It 

 should be noted that the mere fact of the tree growing in 

 summer and resting in winter does not necessarily produce 

 visible alternations in structure. A tree grows in length 



