PINE STEM 139 



1. The cortical tissue bears evident traces of tangential 

 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 composed of three bands 

 (annual rings), in each of which the more central tracheides 

 have large cavity and thinner walls (wood developed in spring) ; 

 passing outwards through the annual ring a gradual reduction 

 of the cavity may be seen, 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. 



III. Cut radial longitudinal sections of a three-years-old 

 stem : mount some in glycerine, others in chlor-zinc-iodine. 

 The section should be accurately radial and longitudinal, 

 otherwise 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. 



2. The xylem, consisting of 



a. Tracheides with lignified walls, and no cell-contents. 

 Starting from those nearest the pith, and passing outwards, the 

 following forms may be observed : 



i. Tracheides with narrow cavity, and more or less regular 



annular or spiral marking the proto-xylem. 

 ii. Elements wider than these, and with bordered pits 



scattered between the spirals. 



iii. Normal tracheides, with bordered pits only : these form 

 by far the greater bulk of the secondary xylem, and 



