190 STORAGE OF FOOD AND WATER 



The phloem part of the medullary rays passes most of the non- 

 nitrogenous foods that come to it over to the xylem part or to 

 the pericycle and cortex for storage, and reserves the bulk of 

 the nitrogenous forms for itself. This fact stands sharply out 

 when a cross section of stem taken in autumn or late summer is 

 placed in a drop of iodine solution. The part of the rays between 

 the phloem strands is then colored yellowish or brown because 

 of its proteid content, and the xylem part is blue or almost black, 

 due to the abundance of its starch. 



The wood parenchyma, which is the predominant tissue in 

 the xylem of many herbaceous plants, and occurs in greater or 

 less abundance in the wood of trees and shrubs, assists the medul- 

 lary rays in the storage of non-nitrogenous foods, and almost 

 the whole of fleshy roots and stems is composed of these two 

 tissues, which there maintain the storage function. 



So long as the medullary rays and wood parenchyma remain 

 living they retain the power and habit of storing food. In 

 trees that have heartwood and sapwood all tissues are dead in 

 the heartwood. In other kinds of trees where heartwood is 

 not formed the medullary rays and wood parenchyma may be 

 alive from bark to pith, even in trees that are fifty or more years 

 old. The rule is, however, that most of the stored food occurs 

 in the younger parts of the wood. 



The phloem parenchyma, like the phloem part of the medul- 

 lary rays, stores up nitrogenous food reserves, and apparently 

 for this purpose it is longer-lived than the other tissues of the 

 phloem, living on sometimes for ten years or more when the 

 sieve tubes and companion cells produced at the same time have 

 long since died. 



The thin-walled parenchyma cells of the cortex and peri- 

 cycle store up both nitrogenous and non-nitrogenous foods, 

 and with them in this the collenchyma is often associated, and 

 altogether they constitute a very significant part of the storage 

 system. 



Fluctuations in the Solubility and Insolubility of Stored 

 Food. When the leaves of woody perennials have finished 



