158 TRANSPORT OF FOODS 



different elements of the phloem and give what is demonstrated 

 or conjectured on good grounds to be their functions. 



The sieve tubes (Fig. 18), it will be remembered, are formed 

 by the perforation of the end walls in vertical rows of cells, 

 so that the row becomes essentially a tube through which all 

 kinds of foods can flow in solution with less interruption than 

 through relatively short cells with walls intact. The sieve 

 tubes are especially well adapted for the conduction of proteids, 

 which are colloidal and diffuse throughout the cell cavity and 

 through division walls with difi&culty. It can be shown that 

 the contents of the sieve tubes can flow en masse through the 

 perforated partitions from one member of the tube to another, 

 for when a stem is cut off the contents of the sieve tubes flow 

 out until the latter are at least partially emptied for one or more 

 internodes back from the cut, and in flowing this distance the 

 contents must in some instances have passed through hundreds 

 of partition walls. 



The companion cells and sieve tubes are formed by the lon- 

 gitudinal division of a common mother cell, and they continue 

 in intimate association. When the walls of the companion 

 cells become appreciably thickened they are frequently pitted 

 where they are in contact with the sieve tubes or surrounding 

 parenchyma or medullary ray cells. The companion cells are 

 therefore adapted to take over materials from the sieve tubes 

 and deliver them to tissues that can carry them where they are 

 wanted for immediate use, or where they can be stored for con- 

 sumption later on. What part the companion cells play in the 

 longitudinal transmission of foods has not been worked out. 



The cells of the phloem parenchyma appear to take an active 

 part in the longitudinal transmission of carbohydrates and 

 amido-acids, and through their pitted communications with 

 the medullary rays they send foods of all kinds into the rays 

 for radial distribution and storage; and they themselves, to- 

 gether with the phloem parts of the medullary rays, are the 

 chief place of storage of proteids during resting periods of winter 

 or dry seasons. 



