124 ANATOMY OF STEM, ROOT AND LEAF 



termed annual-rings and each represents the wood-tissue pro- 

 duced by the cambium during one active vegetative period. 

 From the beginning of one vegetative period to the commence- 

 ment of another is generally one year, so that in a two-year-old 

 stem two rings are visible, in one three-year-old three rings are 

 seen, and so on (Fig. 57). 



It is on account of certain differences between the wood made 

 at the commencement of the growing season and that produced 

 at the end that we are able to recognise these successive yearly 

 additions to the wood as distinct bands, for if the elements pro- 

 duced by the cambium were of exactly similar character through- 

 out its life, it would not be possible to determine the points at 

 which the cambium had ceased or recommenced its growth. 



When the cambium commences growth in spring it gives rise 

 to vessels and cells with thinner walls and wider cell-cavities 

 than those which it manufactures in late summer and autumn ; 

 in each annual ring (r, Fig. 64), therefore, two more or less 

 distinct portions are visible, namely, (i) a layer of spring-wood {s) 

 produced early in the growing season, and (ii) a layer of what is 

 termed autumn-wood (a) produced in late summer and autumn. 



The spring-wood is generally of soft nature and pale colour ; in 

 oak, elm, ash, and Spanish chestnut its vessels are so wide that 

 they appear to the naked eye as a zone of pores. 



The autumn-wood is harder and generally of darker colour ; 

 fewer vessels are present in it, and they are usually too small to 

 be seen with the naked eye. 



The cambium of a stem is continuous with that of its 

 branches, and in a longitudinal section (Fig. 58) the annual 

 increment to the wood of the stem is seen to be continued in 

 the branches, although in the latter the amount added per 

 annum is smaller than in the stem, and consequently the annual 

 rings of a branch are narrower than those of same age in the 

 stem. 



It will be seen from the above Fig. that the basal portions 



