THE PINE 153 



empty wood cells and explain differences in size. Also include in 

 the figure a medullary ray, the cells of which have dense protoplas- 

 mic contents, and also a resin duct. Note the cross sections of pits 

 in the wood cells, which will -be better understood after the study 

 outlined in C. In what faces of the cells are they found ? 



3. Study the region of the cambium and show the form of its cells in 

 a detailed figure, together with some of the wood on the inside and 

 the bast on the outside, including a medullary ray. The bast is 

 composed for the most part of sieve tubes. What are the peculiari- 

 ties of the cambium tissue characteristic of a region of growth ? 



4. Study the outer bark, showing in figures (a) the form of the old 

 bast cells having a crushed appearance ; (6) the green parenchyma, 

 comparing it with the pith ; (c) the manner in which the medullary 

 rays merge with the cells of the bark. 



. Cell structure of the wood. Use the cross sections employed above and 

 also radial (lengthwise) sections and tangential (lengthwise) sections, 

 staining if desired (Sec. 212). 



1. In radial sections note (a) the long, empty wood cells, trackeids, 

 with walls bearing bordered pits (pits characterized by two circles and 

 peculiar to certain groups of gymnosperms) ; (6) the medullary rays, 

 like fong knife blades, piercing the wood, mostly composed of cells 

 with dense protoplasmic contents but some of them empty and 

 pitted. Show these points in a detailed figure. 



2. In tangential sections note (a) the wood cells, tracheids, with cross 

 sections of the bordered pits ; (6) the cross sections of the medul- 

 lary rays. Illustrate in a detailed figure. 



3. Examine the cross sections again to understand clearly the appear- 

 ance of the pits and medullary rays in the light of your study of 

 radial and tangential sections. Make a new figure if the study out- 

 lined in B, 2, is not satisfactory. 



4. Draw a cross section of a pit, showing the delicate membrane, or 

 primitive cell wall (middle lamella), which crosses it, and the second- 

 ary thickening of the cell wall on both sides. 



5. Construct a figure of the appearance of a cube of wood under high 

 magnification, several cells wide, as viewed from an angle so as to 

 show cross, radial, and tangential sections (App. 21). Include in this 

 figure also one or more medullary rays and a resin duct. 



. T?ie cell structure, or histology, of the pine needle. Cut cross sections of 

 a pine needle free-hand (Sec. 194) or use prepared slides (Sec. 212). 

 Observe : 



1. The heavy epidermis, with lengthwise grooves, in which are situated 

 the stomata. 



