NUTRITION 



395 



keeping the latter supplied with food) may Continue to live for years (fig. 665), 

 yet the vigorous growth is above the injury. Girdling experiments with willow 

 shoots are often cited as adequate proofs of the conductive function of phloem. 

 For example, by removing a ring of cortex 5 mm. wide, 

 a few centimeters from the lower end in one case and 

 several times as far in another, and placing both shoots 

 in water, lateral roots and shoots develop in both cases. 

 Their vigor is somewhat proportional to the relative 

 lengths of stem below and above the girdling, and this is 

 taken to indicate that the new parts can draw only upon 

 food stored in the part of the stem above and below the 

 girdling, transfer being prevented by the interruption of 

 the phloem. But if bridges of bark be left across the 

 gap, the differences of development tend to disappear ; 

 and the more numerous the bridges the less the differ- 

 ences. While such experiments agree fairly well with 

 other observations, they are in themselves not con- jj>^ \ 



elusive, since the results are complicated with obscure 

 phenomena of regeneration, and perhaps with wound 

 irritability. 



(4) The content of the sieve tubes, which is a 

 coagulable slime, consists more largely of foods 

 than would be at all likely unless the sieve tubes 

 were organs of either conduction or storage, and 

 the latter supposition is unlikely because the 

 foods are almost entirely in solution. In a typi- 

 cal case analysis showed that, excluding water, 

 the constituents were : carbohydrates, 30 per 



-a 



FIG. 665. Portion of 

 the trunk of a pine, the 

 bark completely destroyed 



cent; amides, 38 per cent; proteins, 20 per cent. b y birds. at a. A single 



c u i r i 1,1 r j uuji circle of branches below 



So rich a supply of soluble foods could hardly keeping all the parts lower 

 be found anywhere else. (5) A bit of merely 

 corroborative evidence is derived from the dis- 



than a scantily supplied 

 with food, the upper part 

 made an excessive growth, 

 tribution and relative development of the phloem, especially in the neighbor- 

 No plants need more facile movement of foods f hoo , d of J , he w und ' but 



food could not pass a 



than vines, whose stems are necessarily slender freely (perhaps not at all). 

 and long, and in none is there better develop- Original in the museum of 



Purdue University. From 



ment of the phloem. Indeed, when the anatomist photograph supplied by 

 wishes to study the largest and most specialized STANLEY COULTER. 

 sieve tubes, vines are almost invariably selected. Moreover, where the 

 requirements for food transfer are the greatest, as in flower clusters and 

 in the branches of inflorescences, the phloem strands are particularly 

 well developed. 



