l6o TRANSPORT OF FOODS 



storage cells on both sides of the girdle ahke. When, however, 

 the ring of bark is removed clear to the wood, so that the possi- 

 bility of longitudinal flow through the phloem is prevented, it is 

 found that growth ceases or is greatly lessened below the girdle 

 and the storage of food below the girdle is prevented. Again, 

 when a willow branch, for example, is cut off, girdled close to 

 its lower end, and placed in a vessel of water, adventitious roots 

 spring out much more abundantly and vigorously above the 

 girdle than below it; and when the girdle is incomplete, so that 

 it is spanned by a strip of phloem, roots spring out in line with 

 the strip above and below the girdle alike. When girdling is 

 performed on plants with, bicoUateral bundles (see page 43), 

 as in the Cucurbitacese, the phloem parts next to the pith are, 

 of course, left intact and the food is found to travel up and down 

 without hindrance. The evidence is therefore conclusive that 

 the phloem is the highway for the vertical transmission of food 

 through the stem. 



In those Monocotyledons where the vascular bundles are 

 scattered promiscuously throughout the stem the phloem can- 

 not, of course, be removed by girdling, since every bundle, 

 near the periphery or remote, consists of both phloem and 

 xylem. 



Evidence that the Tracheal Tissues Assist the Phloem 

 in the Upward Transport of Food. — No tissues of the plant 

 body are so well adapted for the rapid transportation of food 

 upward as the tracheal tissues, consisting of tracheal tubes 

 and tracheids, for in them are currents of water moving much 

 more rapidly than is possible in the phloem. The evidence 

 that these tissues aire so used is found in the chemical analysis 

 of their contents and in the results of girdling. Chemical analysis 

 shows for trees that early in the spring, when a wound results in 

 bleeding, water from the tracheal tissues contains in solution 

 sugar, proteids, and amides. At other times of the year these 

 may be present there but in less amount. The maples and 

 birches are notable examples of this, while the grape is an excep- 

 tion, for the sap gathered from the bleeding of grapevines, after 



