TRACHEAL TISSUES ASSIST PHLOEM 



161 



their pruning in early spring, contains solutes from the soil, 

 but not stored food-stuffs, as in the case of maples, birches, box 

 elders, etc. 



The evidence afforded by girdling that the tracheal tubes carry 

 foods upward in the spring is this: If girdling is done on trees 

 in the fall, winter, or spring, before the resumption of growth sets 

 in, and while wood and bark are still stored 

 full of reserve food, nevertheless the storage 

 cells below as well as above the girdle are 

 emptied of their supplies after the spring 

 growth begins, although the only tissues 

 left capable of carrying the reserve foods 

 past the girdle are the tracheal tissues in 

 the wood (Fig. 87). 



Furthermore, when the main stem of an 

 inflorescence is girdled near its base and 

 the exposed wood is prevented from drying 

 by tinfoil and sealing wax, still the fruit 

 goes on to maturity. In this case also 

 the tracheal tissues afford the only possible 

 channels for carrying past the girdle the 

 large amounts of food needed for the 

 growth of the fruit and for storage in the 

 seeds. 



The inference does not follow from these observations that 

 the phloem is not needed and is not employed in the upward 

 transportation of foods. The only thing proven by the gird- 

 ling experiments is that when the continuity of the phloem is 

 thus broken the tracheal tissues can carry food upward in suffi- 

 cient quantities past the girdle. There are, on the other hand, 

 anatomical facts to show that the phloem is employed in the 

 upward movement of foods. In inflorescences, for instance, 

 which make no food but use large amounts that must be brought 

 upward into them, the phloem, and particularly the sieve tubes, 

 attain to a greater relative development than in any other parts 

 of the plant. 



FIG. 87. Diagram to 

 show path of stored food 

 upward through the tra- 

 cheal tubes, and through 

 the phloem portion of 

 the bark, and showing 

 how this passage is not 

 prevented by girdling. 



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