170 



TRANSPORT OF FOODS 



living cells of the wood in nutrition, or some is stored in the 

 rays and wood parenchyma and cells of the pith immediately 

 bordering the wood (Fig. 94). Some of the food taken from 

 the phloem is distributed outward to the living cells of the cortex 

 and pericycle (see page 31), and used in nutrition or tempo- 

 rarily stored. In short, all living cells of the plant body draw 

 upon the supplies that are in circulation throughout the phloem. 

 Annual Additions to the Food-conducting Tissues. In 

 the spring when growth in perennial stems and roots is resumed 



Tracheal tube carrying food upward 

 Xylem parenchyma receiving 



v food from medullary rays 

 and storing it 



Sieve tube carrying food 

 -- ^^ downwards from 

 yfjVTX the leaves 



H^ 



Medullary ray cells carrying food inward and outward 

 from the sieve tubes The ray cells also store food 



Parenchyma 

 storing food 



FIG. 94. Diagram showing the transport of food through the sieve tubes, medullary 

 rays and tracheal tubes, and its storage in the parenchyma cells of the wood and bark. 

 The black bodies in the cells indicate stored food. 



the cambium devotes itself first to the production of water- 

 carrying tissues, as related in Chapter VI. But at the same 

 time, although to a much less extent, it begins to lay down new 

 phloem elements. Early spring additions to the phloem seem 

 to be more imperative in some plants than in others, because 

 in some the sieve tubes are functional but a single year and the 

 advent of their second spring finds them empty and their sieve 

 plates blocked by an accumulation of a peculiar, highly refractive 

 'substance called callus, and in this condition they remain until 

 crushed out of existence by the growth of surrounding tissues. 

 In other plants, such as the grape, although in the spring the old 

 sieve tubes are empty and dammed up by callus the latter is 

 soon dissolved away and the tubes again are filled from stored 

 materials in phloem parenchyma and medullary rays. 



