164 



TRANSPORT OF FOODS 



form of soluble carbohyhrate such as grape sugar, or in sol- 

 uble nitrogenous form such as asparagin and other amides, 

 and even in the more complex form of soluble proteids, passes 

 by diffusion into the border parenchyma cells of the smallest 

 veinlets, and then begins its movement, still by diffusion and 

 other ways about which we have no exact knowledge, toward 

 the larger veins which are to carry it out of the leaf or deliver 

 it to the midrib for transportation. A very simple demonstra- 

 tion shows that the food takes this course. After a day of active 



Fig. 90. — Diagram indicating the succession of the conducting tissues of a vein from 

 the base toward the apex, a, border parenchyma; b, companion cells; c, sieve tubes; d, 

 undivided mother cells of companion cells and sieve tubes; e, tracheal elements. 



photosynthesis the leaves are loaded with starch at sundown. 

 A leaf removed at this time, bleached in boiling alcohol, and 

 stained with iodine, shows the blue starch reaction all over. 

 By sunrise the next morning, however, a leaf when removed and 

 treated in the same manner takes on a yellowish color, showing 

 that the starch has disappeared. But if one of the principal 

 veins is cut in the evening that portion of the leaf between the 

 cut and the extremity of the vein, which of course would be the 

 part tributary to this vein, still retains its starch in the morning, 

 while the rest of the leaf where the veins are left intact have been 

 emptied (Fig. 91); and when all of the principal veins are cut 

 through at their base the entire leaf is filled with starch in the 

 morning. Clearly the cutting of the veins broke the continuity 

 of the highways through which the food passes out of the leaf. 



The passage of the food through the border parenchyma 

 cells of the smallest veinlets by diffusion must be very slow, 

 but since there is a multitude of these veinlets (approximately 

 6,000 to the square centimeter) tributary to the larger veins 



