COURSE OF FOOD DISTRIBUTION 163 



a tree, there are approximately between 20 and 30 medullary 

 rays in every square millimeter (Fig. 89) ; so that when the leaves 

 are at the height of their food-construction it may be inferred 

 the rays are very active in relieving the sieve tubes of their loads. 

 The Course of Food Distribution.^ — The logical place to 

 begin the discussion of food distribution is where the food first 

 comes into being in the palisade and spongy 

 parenchyma of the leaves. It will be re- 

 membered (see page 115) that the veins 

 ramify throughout the leaf so extensively 

 that the last branches probably average no 

 more than .2 mm. apart; so that food 

 manufactured in cells farthest from these 

 branches, namely, those half way between 

 them, would need to travel laterally about camli-iudd7^o°uTiSrTf 

 . I mm. before entering the border par- tangential section of wood 



of oak, to show frequency 



enchyma cells of the veins in which it of medullary rays. The 

 begins its journey out of the leaf (Fig. 58, C). !ff ^°" ^?, ' "?""■ ^"f "'""• 



" -" -' ^o^'^lhe number of rays shown 



These border parenchyma cells have is below the average for 



, , , 111 1 woodv plants. 



already been told about on page 44, and 



their relation to the other food-conducting cells of the veins 



will now be given more in detail. 



The vascular bundles in the larger veins of the leaf may have 

 all of the elements in their phloem part that occur in the stem 

 from which they spring but as the veins in branching get smaller 

 and smaller the phloem parenchyma is left behind, while the 

 sieve tubes and companion cells remain; then farther along these 

 do not appear and their place is taken by elongated cells that 

 are apparently the undivided mother cells of sieve tubes and 

 companion cells (see page 37); then these are left out, and the 

 ends of the veins have only border parenchyma cells (which are 

 morphologically a part of the mesophyll or fundamental paren- 

 chyma and not of the vascular bundles), surrounding the last 

 tracheids. These facts are represented diagrammatically in 

 Fig. 90. 



The food from the palisade and spongy parenchyma in the 



