VASCULAR BUNDLES 55 
as water-conducting cells, in part probably for support. 
Around these are somewhat elongated thin-walled cells 
that are possibly food-conducting in function. 
78. It is in the higher plants, however, the Ferns and 
Fern Allies and Seed Plants, that a true conducting 
system is developed. This consists usually of strands of 
tracheary and sieve tissue, each associated with some 
living parenchyma cells, passing longitudinally through 
the stems and roots and out into the leaves. These 
strands are called vascular bundles. 
79. A vascular bundle consists of two parts which are 
distinguished both structurally and functionally. Xylem 
is the name given to that part of a vascular bundle 
consisting of the tracheary tissue and the parenchyma 
associated with it. Its function is primarily water- 
conducting. The phloem, on the other hand, consists 
of the food-conducting sieve tissue, with the accom- 
panying parenchyma in the form of companion cells, 
sieve parenchyma, etc. Frequently fibrous tissue is 
found intimately connected with the xylem and phloem, 
usually in the form of wood fibers with the former and 
bast fibers with the latter. In such a case, we find the 
supporting system to be partially united with the 
conducting system. 
80. The vascular bundles originate in the growing 
points by the conversion of certain of the rows of meris- 
tem cells into strands of elongated, rather narrow cells. 
These, beyond elongating considerably and dividing 
longitudinally so as to become narrow, retain their 
meristematic character long after the surrounding 
tissues have acquired the more permanent forms. 
They are then known as procambium or as procambial 
strands. Eventually, the cells composing them begin to 
change into the permanent tissues, these changes 
