stout: pollinations in cichorium intybus 439 



is so great that the excess supplied to pistils through the transpira- 

 tion current of plants in wet seasons may be a cause of marked 

 sterility in this species. It does not appear, however, that this 

 is the actual cause of the self-sterility which seems to prevail in 

 this species, for self-sterility is in evidence even under the condi- 

 tions most favorable for cross-fertilization, and the marked sterility 

 of a plant in the first crop seems to be more closely related to the 

 impotence of pistils in that its tissues remain vegetative. Later in 

 the season the same plant may produce a new crop of flowers with 

 functional pistils but with the plants highly self-sterile_ from 

 physiological incompatibility but highly cross-fertile as pollinated 



in the field. 



Tokugawa's recent study gives no data that are directly con- 

 cerned with the problems of self-sterility, although he worked with 

 the pollen of several species known to be self-sterile. His results 

 relate chiefly to a comparison of pollen-tube growth in cases of 

 rather wide crossing and give no clue to the causes of self- and 

 cross-sterility in such cases as chicory and Cardamine. 



Favorable or unfavorable growth of pollen tubes may, to a con- 

 siderable degree, depend quite as much on quantitative as on 

 qualitative relations, not only in respect to such incompatibihties 

 as are shown in the precipitation reactions but in the case of stimu- 

 lating or inhibiting substances. Miyoshi ('94) has emphasized 

 this point in presenting data on pollen-tube growth, which show 

 that to divert tubes from one solution to another it is necessary to 

 increase greatly the concentration of the second, as would be 

 expected from Weber's law. 



There can be no doubt that in the same strain different plants, 

 and even different pistils on the same plant, possess variations in 

 the total and relative amounts of materials that may serve for 

 the nutrition and stimulation of pollen tubes, and that pollen grains 

 possess considerable variations in their own supply of nutritive 

 materials and in their requirements for growth. Such variations 

 are operating in all cases of self-fertility. The causes of self- and 

 cross-sterility are, however, quite independent of these. 



The final stages of growth of pollen tubes in pistils should be 

 more adequately determined with reference to the relative de- 

 velopment of the macrogametophyte and the egg. It is highly 

 essential to know the degree of development of the macrogameto- 



