PINUS. STEM. 145 



transition to the spring wood. At this point is the 

 limit of each year's growth. 



IY. Cut radial longitudinal sections of a three years' 

 old stem. Mount some in glycerine, others in Schulze's 

 solution. 



The sections 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 out- 

 wards observe successively : 



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

 of which are of parenchymatous form. 



a. Cells with cellulose walls (blue with Schulze's 

 solution) pitted, with protoplasm and nucleus. 



1. 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. 



2. The Xylem consisting of 



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

 tents. Starting from those nearest the pith (i.e. from 

 the protoxylem), and passing outwards, the following 

 forms may be observed, and distinguished mainly by 

 the markings due to unequal thickening of the walls. 



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

 regular annular or spiral marking. 



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 must be carefully studied. Their form 

 is prosenchymatous. The greater part of the 



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