366 



PLANT GROWTH SUBSTANCES 



series of hormonal stimulations beginning with the auxin released in the 

 developing gametophytes and ending with its production and accumula- 

 tion in the developing fruit. The effects of the release of these natural 

 growth substances during various phases of reproduction are frequently 

 confined not alone to stimulating fruit setting and growth but are 

 extended to vegetative parts as well, exerting a profound effect upon 



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Figure i. Changes in the growth hormone content of the reproductive 

 organs of the corn plant during their development. There is complete absence 

 of growth substance in the male inflorescence prior to synapsis, and in the ovule 

 before fertilization. Subsequent to chromosome conjugation in the tassel, and 

 the union of gametes in the ear, growth hormones appear in considerable 

 quantities in these structures. 



the growth and metabolism of the entire plant (53). Such mechanisms of 

 auxin action involving intact higher plants are complicated, indeed, and 

 can hardly be explained in terms used for describing auxin effects on 

 excised tissues (2). 



From the number of independent observations reported recently, 

 there can be little doubt that the immediate causal factors in fruit setting 

 are hormonal, providing nutrition is adequate. Considerable confusion 



