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 S3'stem 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 di^^ding 

 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 



