122 GROWTH HORMONES IN PLANTS 



Stomatal Movement. — In an attempt to find out whether 

 growth substance influences the production of starch in the 

 guard cells and therefore possibly affects the opening of the 

 stomata, Boysen Jensen removed the petioles of leaves of Sinapis 

 and Sambucus and placed them in water and in growth-substance 

 solutions of different concentrations. Although the growth- 

 substance solution was taken up by the leaves in abundant 

 amounts, no effect upon the degree of opening of the stomata 

 could be distinguished, nor could any increase or decrease of 

 starch in the guard cells be observed. 



Respiration. — Boysen Jensen and Nielsen (1925) published 

 some experiments dealing with the effect of decapitation on the 

 intensity of respiration in Avena coleoptiles. It was found to be 

 practically the same in decapitated and nondecapitated coleop- 

 tiles, and no effect of growth substance upon respiration could 

 be shown. Nielsen and Hartelius (1932) also found that rhizopin 

 was without effect in this respect. According to the more recent 

 experiments of Bonner (1933a, h), however, growth substance 

 may influence the intensity of respiration of the Avena coleoptile. 

 Coleoptile cylinders 3 mm. long were placed in growth-substance 

 solution, and the intensity of respiration was determined. In 

 the lower concentrations the rate was promoted about 27 per 

 cent; and at higher concentrations there was a retardation. 

 When the growth substance was inactivated by treatment with 

 peroxide, no promoting effect could be observed. 



Biinning (19346) concluded from his observations on Aspergillus 

 that the assimilation of nitrates was increased by the promoting 

 effect of growth substance on respiration. The effect was 

 brought about by moving the pH in an alkaline direction, which 

 increased conidia formation. 



Cambial Activity. — The stimulating influence of foliage leaves 

 upon cambial activity has been suggested by a number of workers 

 (Coster, 1927; Thoday, 1933). Kastens (1924) had suggested 

 that the stimulus for cambial activity may be a hormone. The 

 observations of Snow (1933) made it seem probable that the 

 stimulus contributed by developing buds might be some chemical 

 substance in the nature of a hormone. Snow and LeFanu (1935o) 

 obtained indications of increased cell division in the cambium of 

 young Helianthus plants which had been decapitated and treated 

 with urine extracts. In more detailed experiments these same 



