stout: pollinations in cichorium intybus 437 



securing germination had been considered as an evidence of highly 

 developed specificity in requirement (Jost '05, '07; Martin '13; 

 Tokugawa '14). 



It has been shown that pollen tubes of a species often make best 

 growth in and show a marked chemotropism to a particular sub- 

 stance, and that in cultures usually a particular concentration 

 gives the best growth. The limitations and uncertainties of the 

 methods of artificial culture are most evident. The best results, 

 as Jost points out, show a rather feeble growth ; usually the tubes 

 only reach a length that does not exceed a few diameters of the 

 pollen grain. In corn, for example, the best growth reported by 

 Andronescu ('15) was equal to 3-5 diameters of the pollen grain 

 and was therefore only a very small fraction of the length of the 

 style and stigma of corn. Many cases reported as germination may 

 not be due to real growth but simply to swelling by tugor. The 

 results of studies in germination are most often of questionable 

 value owing to the lack of an adequate criterion of what constitutes 

 natural germination and growth. 



The best growth reported by Adams ('16) in his studies of the 

 germination of pollen of the apple is 1336 (x and of black currant 

 is 688 /x, which in length is about 30 and 12 times the diameters of 

 the respective pollen grains. Under the microscope this appears 

 to be a vigorous growth, but when we consider that the growth is 

 only 1 3^ millimeters in length it is, in comparison, hardly sufficient 

 to do much more than penetrate the stigma. The limitation of 

 such results in explaining process of pollen-tube growth in the 

 intricate relations of fertility and sterility is apparent. 



In pistils themselves it has been shown that pollen tubes advance 

 by mechanically penetrating through the pistil or by following 

 stylar canals, that they may (in experiments) grow in the reverse 

 direction from that which is natural, and that there is no marked 

 difference in culture tests between the chemotropic power of 

 different sections of the pistil. 



Nearly all investigators have agreed that the growth of the 

 tubes into the micropyle must be influenced by some sort of secre- 

 tions of the egg itself, but it is not fully established just what this 

 influence is or to what distance it acts. 



The difficulty of establishing the fundamental facts regarding 

 the relations of pollen-tube growth to the secretions and cell 



