210 PLANT PHYSIOLOGY 



nearest it is the first to be freed from the inhibiting effects of the 

 hormone, it is the first to grow out, producing new hormones 

 (i. e., inhibitors), which then check the development of the other 

 buds. The degree of inhibition depends upon the size of the bud 

 and the amount of hormone produced, so that under certain cir- 

 cumstances the inhibition extends only to those primordia near 

 the apex. 



On the basis of the hormone hypothesis Swingle (1926) has 

 explained the fact that date pollen influences not only the char- 

 acter of the date seed, which may be explained by xenia, but also 

 influences the size, shape, character, and time of ripening of the 

 fruit. He believes that the embryo or endosperm of the date seed, 

 or both together, constitute a ductless gland apparatus which by 

 secreting hormones affects the development of the entire date 

 fruit. Montemartini (1929) found that when a red-hot wire was 

 thrust into an unripe pear, penetrating and killing the seed, the 

 ripening of the pulp was greatly accelerated. This also is inter- 

 preted as the effect of a hormone which diffuses out from the seed 

 into the fruit. 



Likewise Went (1926) has explained the phototropic response 

 of oat seedlings as due to a photoreactive hormone which is pro- 

 duced at the tip of the coleoptile and descends on all sides. Its 

 function is to increase the elasticity of the cell walls and permit 

 the cell to increase in size (grow). Light checks the descent of the 

 hormone, so that when illuminated even an extremely small 

 amount, the hormone passes down only on the shaded side, where 

 growth then occurs, with the result that the plant bends towards 

 the light. The hormone is thus seen to be a "growth hormone" 

 rather than a "tropohormone." These results have been confirmed 

 by Soding (1925-1929), who also found that when decapitated 

 coleoptiles were reattached by a gelatin bridge, growth was more 

 rapid than when the tips were not replaced. When peduncles of 

 Cardamine and Cephalaria were cut off and reattached by agar, 

 similar results were observed, which indicate that there is some 

 substance, i. e., a hormone, which can cross the agar or gelatin 

 bridge and affect the growth of the parts below. 



Boysen-Jensen and Paal have reported like results, and Snow 

 found that the geotropic stimulus in Vicia faba can be transmitted 

 across protoplasmic discontinuities. Perry (1932) found that the 

 coleoptiles of grasses in general and of oats in particular contain 



