Ill THE MEASUREMENT OF GROWTH 755 



weight. The aerial sporangiophores of fungi can, of course, be treated like the 

 shoots of higher plants; their elongation is limited to a very narrow zone. For 

 example, in Phycomyces sporangiophores 60 or 70 mm long, the growing zone is 

 less than 2 mm in length. 



With these simple methods the facts of growth have gradually been uncovered, 

 and work continues actively in many directions. Attempts to reduce growth phe- 

 nomena to simple mathematical relations, popular in the 1920's, have receded 

 today, as it has become clear that they only describe the process and do not help 

 in its analysis. "Autocatalytic" curves, the Compound Interest law, Huxley's "He- 

 terogonic growth" formula, and the like, often approach closely to a representation 

 of a given growth process but are seldom useful in analyzing it. Empirical relations 

 are more informative, providing they are obtained under rigidly repeatable 

 conditions. 



{b) The use and measurement of auxins 



The discovery of auxins and, more recently, of other plant growth factors, 

 sharpened and gave meaning to growth measurements and made it possible to 

 analyze them in causal terms. The history of the discovery of auxins has been 

 presented several times and will not be repeated here. Full treatments are given in 

 Went and Thimann, 1937, Chapter 2 and parts of Chapters 7, 9 and 10, and in 

 Boysen-Jensen, 1936, Chapter i. The important point to note is that the method by 

 which it was finally proved that a diffusible growth substance exists and can be 

 separated from the plant became the basis for a quantitative bioassay. For, in an 

 attempt to isolate the growth substance whose presence was indicated by a great 

 deal of indirect evidence, especially in the Avena coleoptile. Stark, in 1921, 

 extracted various tissues, mixed them with agar and applied the resulting agar, 

 in little blocks, to one side of decapitated coleoptiles. Had they contained a growth- 

 promoting substance, he reasoned, (from the behavior of isolated tips studied 

 earlier by Paal) then the side to which the blocks were applied would have grown 

 more than the opposite side, and a curvature would have resulted with the block 

 on the convex side. Although Stark's attempt failed, this indeed was what was 

 actually observed by Went (1928), after he placed living isolated coleoptile tips 

 on agar blocks, so that they continued to excrete auxin into the block. Went then 

 made the method quantitative by measuring the angle of curvature. This Avena 

 test has been widely used, especially in the more botanical aspects of growth 

 work. It has been modified by several workers and details of the final form need 

 not be given here (Went and Thimann, 1 937, Chap. 3 ; Thimann et al., 1 952, Chap. 2 

 and Leopold, 1955, Chap. 2). In essence the seedlings are grown to a standard age 

 (about 74 h. at 25°) in small glass holders in darkness or weak red light, the extre- 

 me tips of the coleoptiles removed, and the plants left 3 h. for their internal auxin 

 level to decrease to a minimum. Then a further 4 mm of the remaining coleoptile 

 tissue is removed, the primary leaf inside is pulled partly out, and a small (ca. 10 

 mm3) block of i .5% agar containing the test substance is placed on the cut surface, 

 resting against the primary leaf. The resulting curvature reaches its maximum at 

 90-110 min. after applying the block; the plants are photographed or shadow- 

 graphed and the curvatures measured at leisure with a simple goniometer. They are 



Literature p. 816 



