26 STEMS 
will be room next to each, for drawing of longitudinal 
sections of similar tissues. Ask the instructor to correct 
your drawings on the histology of the stem, while the 
section drawn is still under the microscope. The cavities 
of some of the cells in the green bark contain numerous 
granules calied chloroplasts. How do the cell walls of 
these cells compare in thickness with those found in the 
brown bark? Draw three or four cells and fill in one of 
them. In the green bark near the wood tissue you will find 
patches of light colored cells, bast fibers. These cells are 
ciosely packed together. ‘They are smaller than the green 
cells, have thick walls, and cavities so smali that they 
appear like mere dots under the microscope. Draw three 
or four. Between the wood and bark is found a thin 
layer of tissue composed of thin flat cells. This layer is 
known as the cambium region. It can be most clearly 
seen in rapidly growing stems because the enlargement 
of the stems takes place in this region. The instructor 
will demonstrate how growth takes place, to the class 
as a whole. Draw a few cambium cells. 
The woody tissue is composed of three kinds of 
cells, wood-fibers, which appear in cross-section some- 
thing like bast fibers; vessels, containing large cavities 
which appear like definite holes thru the section; and 
medullary ray cells, which are much like pith cells, con- 
siderably elongated. Draw two adjoining vessels so as 
to represent the relative thickness of their walls, three 
or four wood fibers and medullary ray cells, and also 
afew pith cells. Test the cells in the stem for starch and 
cellulose. In which do you find starch; in which 
cellulose? 
