Growth Substances 379 



plants. He also showed that fruits which are naturally parthenocarpic 

 have a much higher concentration of auxin than do seed-bearing ones. 

 In some cases parthenocarpy does not require application of a growth 

 substance to the unfertilized ovary but simply the presence in the air of 

 the greenhouse of vapor of a specific substance. Induction of partheno- 

 carpy is by no means universally possible and has been found more 

 frequently in members of the Solanaceae and Cucurbitaceae than in 

 other families. The subject has been reviewed by Vazart (1955). 



Fig. 18-3. Parthenocarpy. Left, normally pollinated tomato fruit with many seeds. 

 Right, parthenocarpic fruit produced by treatment with synthetic growth substance. 

 ( Courtesy Boyce Thompson Institute. ) 



Davidson (1950) found that several marine algae were stimulated in 

 their growth by synthetic substances in the sea water (Fig. 18-4). Al- 

 though auxin seems rarely to be concerned with the growth of fungi, 

 Fraser (1953) reports that indoleacetic acid stimulates growth in the 

 common mushroom. 



Cell Enlargement. The primary effect of auxin on plant growth seems 

 to be its promotion of increase in cell size, especially in the stem and 

 in its longitudinal dimension. In phototropism the side of the axis away 

 from the light grows faster than the lighted one, and Went ( 1928 ) and 

 others showed that auxin is more abundant on this shaded side. Avery 



