PLANT TISSUES 135 
companion cells and sieve tubes arise by the division of the same 
mother-cell of the procambium. ‘The companion cells occur in the 
phloem of Angiosperms only. In cross sections they appear as 
small, rounded or angular cells at the corner or lateral to a larger 
sieve tube. In longitudinal section they are seen to be elongated, 
slender cells. They may be distinguished from the mature sieve 
tubes by their abundant granular, cytoplasmic contents, their 
small vacuoles, and also by the fact that they retain their nuclei 
after complete maturation. Their walls are pitted where they 
are in contact with sieve tubes with which they are associated, 
phloem parenchyma and medullary rays. ‘Their work is to take 
over food materials from the sieve tubes and pass them on to 
other tissues needing them for growth, repair or storage. 
Besides sieve tubes and companion cells, phloem fibers also 
called bast fibers, and parenchyma cells called “‘phloem-parenchyma”’ 
or “‘steve parenchyma’’ are often found in thephloem. The phloem- 
parenchyma cells transmit soluble carbohydrates, amino-acids 
and amides slowly downward and, through their communication 
with medullary rays, deliver food materials to them for radial 
distribution and storage. 
TRACHEARY ‘TISSUE 
The tracheary tissue of plants comprises two kinds of elements, 
the trachee (ducts or vessels) and tracheids. They are formed first 
by the procambium or plerome and later by the cambium. 
Both of these conduct crude sap (water with mineral salts in solu- 
tion) upward through the roots and stems into the leaves. The 
trachee are very long tubes of a cylindrical or prismatic shape 
which are formed by the breaking down of the transverse walls 
between vertical groups of superimposed cells, during the growth 
of the plant. Thousands of these trachea are arranged one 
above the other from the smallest branches of the root to the 
highest part of the stem and out into the farthest reaches of the 
leaves. Cross or end walls which are perforated with openings 
separate each trachea from its neighbors. In this way each 
line of trachez serves as a continuous tube for the conduction 
of water rapidly from root to leaf. Most trachee have two 
perforations, one in either end but as many as four have been 
