PINE STEM 230 



tial extension. This is necessary to keep pace with the 

 increase in bulk of the vascular system. 



2. The phloem is thicker, and the constituents of the 

 outer part of it are much distorted and displaced. 



3. The xylem has increased in thickness more than 

 any other tissue, so that it is now the chief constituent 

 of the stem. It may be distinguished as being com- 

 posed of three bands (annual rings), in each of which 

 the more central tracheides have large cavity and thin- 

 ner walls (wood developed in spring) ; passing outwards 

 through the annual ring there may be seen a gradual 

 reduction of the cavity, and increase in thickness of the 

 walls till a certain limit is reached (autumn wood). 



Outside the latter is a sudden transition to the 

 spring wood : at this point is the limit of each year's 

 growth. 



IV. Cut radial longitudinal sections of a three- 

 years-old stem : mount some in glycerine, others in 

 chlor-zinc-iodine. 



The sections should be accurately radial and longitudinal, other- 

 wise the difficulty of study of the tissues is greatly increased. 



Beginning at the centre of the stem and passing- 

 outwards, observe successively : 



1. The pith, consisting of two sorts of elements, both 

 of which are of parenchymatous form. 



a. Cells with pitted cellulose walls, and having 



protoplasm and nucleus. 



b. Elements of similar form with pitted lignified 



walls, and no cell-contents. 



From the arrangement of these it may be concluded 

 that they had a common origin. 



