IX THE HISTOLOGY OF THE SHOOT AND ROOT 107 



by the action of the cambium and each ring marks a years 

 growth. 



Each annual ring consists of a dark coloured layer and a 

 light coloured layer. In spring, . when the active period of 

 growth commences, the pressure on the cambium is very little, 

 because during the winter the bark and cortex have been 

 ruptured by the action of frost and changes in temperature. 

 The cambium is able to produce large cells and vessels which 

 are thin- walled, thus a light coloured layer is formed. During 

 spring the ruptures 

 in the bark are re- 

 paired, and as the 

 season advances, 

 more and more 

 pressure is brought 

 to bear on the cam- 

 bium, and smaller 

 and thick - walled 

 cells and vessels 

 are formed. These 

 are dark in colour 

 because they con- 

 tain less air, and 

 thus a dark colour- 

 ed layer is formed. 

 The age of a tree 



can be told by its annual rings, and if the rings are examined 

 and compared the size of the layers will give us some clue to 

 the kind of season when any ring was produced. 



The Formation of Periderm. In those plants which 

 grow in thickness the epidermis is replaced by a new tissue 

 which receives the name of periderm. The periderm is 

 formed from the pericycle, which divides up into a number of 

 rows of cells ; one of these rows forms the phellogen or cork 

 cambium. The phellogen produces new cells on both its 

 inner and outer surfaces ; the cells on the inside keep their 

 living contents and form the phelloderm ; those on the outside 

 lose their living contents and their cellulose walls are converted 

 into cork. The cork cells are impervious to water, and so cut 

 off the supply of water to the cortex and epidermis ; these con- 



FIG. 133. Section of Larch stem ; showing annual rings. 



