172 TRANSPORT OF FOODS 



procambium of the succeeding year's growth in length. It will 

 be seen by this diagram that the food highways in the leaves 

 have direct communication with the primary phloem in the new 

 segment of stem which bears them, and this primary phloem is 

 in turn continuous with the phloem elements which the cambium 

 builds the current season throughout branch, trunk and roots. 



As has been said, in some plants the sieve tubes are functional 

 for one year only; in others for two or more years, and it would 

 therefore depend upon the kind of plant to what extent the 

 phloem strands in the older annual segments (2, 3, of the dia- 

 gram) assist in carrying food the current season. It is precisely 

 because the youngest and younger phloem tissues are the most 

 active that, in girdling, the bark can be stripped off nearly to the 

 wood without apparently hindering the flow of food. 



Character of the Food While in Transport. — The nitrog- 

 enous foods circulating in the sieve tubes and other parts of 

 the phloem are mostly in the form of asparagin and other amides 

 which are soluble and more diffusible than the soluble proteids. 

 The insoluble proteids must be changed to the soluble condition 

 before they are capable of translocation. 



The carbohydrates are mostly in the form of glucose (grape 

 sugar) , but saccharose (cane sugar) is sometimes present. Minute 

 starch grains frequently occur in the sieve tubes; and in some 

 plants the pores in the sieve plates are large enough for the 

 smallest grains to pass through; but it is certain that no signifi- 

 cant amount of carbohydrate circulates in this form. 



Oils can be absorbed into the phloem elements in the form 

 of a fine emulsion, and in this form they can travel longitudi- 

 nally through the sieve tubes and parenchyma cells. Chem- 

 ical analysis shows, however, that very little non-nitrogenous 

 food travels in this form, oil for the most part being transformed 

 into sugar preparatory to translocation. 



The Propelling Power in Food Transport. — The neces- 

 sary conditions are always present for the distribution of foods 

 in solution by diffusion throughout the length of the phloem, 

 and foods must circulate to a certain extent in this manner; but 



