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 limes 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- 

 clusive, 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 

 cent; amides, 38 per cent; proteins, 20 per cent. 

 So rich a supply of soluble foods could hardly 

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

 corroborative evidence is derived from the dis- 

 tribution and relative development of the phloem. 

 No plants need more facile movement of foods 

 than vines, whose stems are necessarily slender 

 and long, and in none is there better develop- 

 ment of the phloem. Indeed, when the anatomist 

 wishes to study the largest and most specialized 

 sieve tubes, vines are almost invariably selected, 

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

 in the branches of inflorescences, the phloem strands are particularly 

 well developed. 



Fig. 665. — Portion of 

 the trunk of a pine, the 

 bark completely destroyed 

 by birds at a. A single 

 circle of branches below 

 keeping all the parts lower 

 than a scantily supplied 

 with food, the upper part 

 made an excessive growth, 

 especially in the neighbor- 

 hood of the wound, but 

 food could not pass a 

 freely (perhaps not at all). 

 ( higina] in the museum of 

 Purdue University. — From 

 photograph supplied by 

 Stanley Coulter. 



Moreover, where the 



