THE STEM. 31 
tions, and is practically impervious to moisture, though not to 
gases. Protoplasm and cell-sap occupy the interior of the epi- 
dermic cells, and chlorophyll granules are also found there. Most 
cells of the kind do not contain such granules. The hairs appear 
as large multicellular structures growing from the epidermis, and 
small openings, known as stomata are present. These are more 
abundant in leaves, and will be spoken of in connection with those 
members (p. 65). They are never found in roots. The ground- 
tissue is mainly composed, as in the root, of parenchyma (¢/. 
p- 14), with cellulose walls and protoplasmic contents; but the 
parenchyma of the cortex differs from that of the root in the pos- 
session of numerous chlorophyll granules. There is, besides, in 
the cortex another kind of tissue, collenchyma, forming a band 
beneath the epidermis. This differs from parenchyma mainly in 
the fact that the cells are thickened at their angles. Numerous 
chlorophyll granules are imbedded in their protoplasm. It is 
best to examine the vascular bundles in sections of a very young 
stem, as they are then quite distinct from one another.! In a 
properly stained cross-section (¢f Appendix) we shall be able, 
with a low power of the microscope, to distinguish the bundles as 
oval areas, each of which is composed of an outer part, the bast 
or phloém, and an inner part, the wood or xylem, separated by a 
band of very thin-walled cells known as the cambium. As seen 
with a high power (fig. 6), the bast is made up of an outer some- 
what crescentic portion, the hard bast, the elements of which, 
bast fibres, have very thick walls and no protoplasm, and an inner 
part, the soft bast, the components of which have thin cellulose 
walls. Some of these, the szeve tubes, are large, others, making 
up the bast parenchyma, are much smaller. A number of the 
sieve tubes present an appearance reminding one of the perforated 
top of a pepper-castor. This appearance is due to the presence 
of sieve plates pierced with small holes. The most conspicuous 
components of its wood are the wood vessels, large rounded ele- 
ments with very thick walls and no protoplasm. The remaining 
parts are wood parenchyma and wood fibres, the latter closely 
resembling the bast fibres. The cells of the cambium have very 
thin cellulose walls and abundant deeply-stained protoplasm. 
Their shape is rectangular, with the long diameter tangential, 
and they are arranged in radial rows, two or three cells in a row. 
These features show that division has taken place in a tangential 
direction. Imagine a cell, square in cross-section, with two sides 
at right angles to a radius of the stem. Then suppose this cell 
to be bisected by formation of a new wall parallel to the two in 
* In buttercup and some other plants the bundles remain distinct through- 
out life. 
