STRUCTURE OF MONOCOTYLEDONOUS STEMS 189 
they do not collapse under the pressure of surrounding tissues, 
may form rings as in annular vessels, spirals as in spiral vessels, 
or be more generally distributed over the wall, leaving only 
small unthickened areas which constitute the pits characteristic 
Fic. 167. — Cross section of a vascular bundle of Corn highly magnified. 
s, strengthening tissue; p, phloem consisting of sieve vessels (e) and companion 
cells (c); xz, xylem consisting of annular vessel (a), spiral vessel (h) and pitted 
vessels (¢); b, parenchyma cells. 
of pitted vessels. The xylem vessels are free from protoplasm 
and are composed of cells joined in series with end walls resorbed. 
They are known as tracheae, and are quite tube-like in struc- 
ture and function. Through them the water and mineral salts 
from the roots are carried, some reaching the leaves and buds 
while much leaks out through the cellulose portions of the walls 
to supply the tissues of the stem. Around the vessels are the 
thin-walled parenchyma cells which may function some in con- 
duction.. 
In the phloem there are sieve vessels and companion cells. The 
sieve vessels are composed of cells joined in series and so named 
because of the perforated areas occurring in their end and side 
