PINUS AUSTR1ACA, OR P. LARICIO. 133 



MINUTE STRUCTURE. 

 I. THE STEM. 



Make a transverse section of a year-old stem, 1 collected in 

 May or June, and study the different tissues composing it as 

 follows: 



1. The pith, occupying the center of the section. Observe: 



a. The general outline of the region. In some sections 

 will be seen portions of the pith that run outward. These 

 lead into branches. 



b. The form and arrangement of the cells. 



c. The contents of cells; test for starch. 



2. The wood (xylem), the heavy- walled tissue surrounding 

 the pith, and separated somewhat regularly into wedge- 

 shaped masses by the medullary rays. 



Select a good wedge and observe: 



a. The resin-ducts, one or two of which appear at the 

 inner edge of the wedge. Immediately around the 

 duct is a circle of very thin cells, the secreting layer. In 

 each of these cells is the granular nucleus, characteristic 

 of secreting cells in general. Surrounding the secreting 

 layer and much more prominent is a layer of thick- 

 walled cells, forming a sheath. 



b. Between the resin-ducts and the pith a few very small 

 rounded cells with rather thick walls, the primary xylem 

 vessels. 



c. The main bulk of wood fibers, the tracheids. In the 

 wood observe: 



i. The form and arrangement of the tracheids. 

 ii. Their emptiness. 



3. In the thinnest part of the specimens search for sections 



1 Sections may be made of stems by using material that has been pre- 

 served in alcohol for at least a few days, then soaked in a mixture of 

 alcohol and glycerin. Such sections may be used directly or may 

 be rendered somewhat more satisfactory by use of differential stains. 



