44 
PLANT MORPHOLOGY. 
attain the length of the latter. They are not infre- 
quently branched at the ends, and besides a thin pro- 
toplasmic layer, they usually have no other contents 
than water and air. They usually have the yellowish 
color, characteristic of stone cells, and also a similar 
lamellation and refraction of the wall. 
III. CONDUCTING CELLS. 
Conducting cells include those cells which are chiefly 
concerned in the transferral of either crude or assimi- 
lable food materials. The more or less crude inor- 
ganic materials are carried from the root through the 
woody portion of the stem to the leaves, and from the 
leaves the products of C0 2 assimilation, as well as 
other plastic substances, are distributed through some 
of the tissues of the bark to other parts of the plant. 
The tissues or elements of the wood which conduct 
food materials are of several forms and include tracheae 
or ducts, tracheids and conducting parenchyma; and 
the elements of the bark which transport the assimi- 
lable materials, comprise the sieve and conducting 
parenchyma, although there are other elements which 
sometimes assist these two groups of cells in the work 
of conduction. 
The tracheae or ducts are formed by the disintegration 
and removal of the transverse walls between certain 
superimposed cells, forming an elongated cell or tube, 
which occasionally retains some of the transverse 
walls. The longitudinal walls are relatively thin and 
consist of lignocellulose, giving more or less pro- 
nounced reactions with phloroglucin and aniline 
sulphate. 
The thickenings of the longitudinal walls of ducts 
are quite characteristic, several forms being distin- 
guished : Those having the thickenings in the form of 
