THE STEM 101 
tion, their arrangement suggests a grotesque resemblance to 
the face of an old woman wearing a pair of enormous specta- 
cles and surrounded by a cap frill of netting with very wide 
meshes. These are parenchyma cells, f, f, Fig. 115, and 
constitute the greater portion of the living tissues. 
The two large openings, m, m’, that represent the spectacles, 
are ducts for carrying water up the stem. They are called 
pitted ducts on account of the bordered pits which cover 
their outer surface. The two smaller openings between and 
slightly below the pitted ducts are also vessels for carrying 
liquids up the stem. The lower one, a, is called the annular 
tracheid because its tube is strengthened by rings on the 
inside. The upper, smaller one, sp, is known as the spiral 
tracheid, because its walls are reinforced by spiral thickenings. 
Can you think what is the use of these strengthening contri- 
vances in the walls of conducting cells? (Suggestion: What 
is the use of the spiral wire on a garden hose?) The large, 
irregular opening below the ducts is an air space. What is 
its object? Why has it no surrounding wall? 
Next look above the ducts for a group of rhomboidal or 
hexagonal cells, v, v, with smaller ones, s, between them. The 
larger of these are sieve tubes, the smaller 
ones, companion cells. The sieve tubes 
carry sap down the stem after it has been 
made into food by the leaves. They get 
their name from the sievelike openings 
between the connecting walls of the cells 
which form them — as if a row of pepper 
boxes with perforations at both top and __, Fis. 117.—Horizon- 
tal view of the sieve tube 
bottom were placed end to end, so as to of agourd stem, showing 
form a long tube divided into compart- Pefortions. 
ments by perforated walls. Can you give a reason why the 
cells of ducts that carry elaborated nutriment should have a 
more open line of communication than those carrying crude 
sap? [56 (2).] Which one of the organic food substances was 
shown by Exp. 39 to be unable, or nearly so, to pass through 
