Kirk wood : Pollen-tube in Cucurbitaceae 337 



tions are reversed, and starch is entirely absent from the ample con- 

 ducting-tissue, though abundant in the pollen-tube almost to the time 

 of its [entrance into the embryo-sac. These facts are regarded as 

 evidence that the directive stimulant, in these cases at least, is of a 

 different character from the substances in the tube. It is f however, 

 recognized that the absence of starch from the cells of the con- 

 ducting-tissue may not mean an absence of soluble carbohydrates, 

 for no starch is visible in the secreting cells of some nectaries. The 

 application of iodine to sections of the style and ovary of Cucurbita 

 easily brings out the pollen-tubes in contrast with the conducting- 

 tissue, the former rich in starch, coloring a dark blue, the latter a 

 deep yellow, indicating an abundance of albuminous material. The 

 pollen-tube branches in the nucellus according to the disposition 

 of starch, as above cited in reference to Longo's work. 



If the growth of the pollen-tube or its branches is thus regu- 

 lated by the occurrence or distribution of nutritive material, we 

 have apparently an explanation for the growth of the tube through 

 the nucellus to the embryo-sac, structures which, in the cases here 

 considered, are normally filled with starch before the approach of 



the pollen-tube. 



At what distance from the ovule the pollen-tube comes under 

 its influence, has not in these cases been determined, but it is evi- 

 dent that the embryo-sac is the source of a stimulant of some sort 

 by which the pollen-tube is directed unerringly toward it. As to 

 the character of this stimulant the evidence is meager, but what 

 there is seems to indicate a sugar. 



Miyoshi's results with the pollen of Digitalis and numerous 

 other plants ' show that their tubes are sensitive to the presence 

 of certain soluble carbohydrates, but are indifferent to or repelled 

 by a variety of other substances. Moreover, the fact cited else- 

 where by the same author to the effect that the pollen-tubes of 

 distantly related plants were attracted to the same ovule and to 

 different ovules would indicate that the stimulant is some substance 

 quite common in plant tissues. 



It is suggested here that a difference in the degree of concen- 

 tration of such a solution as a sugar may be a sufficient factor in 

 directing the pollen-tube. This is in accord with the observations 

 of Miyoshi and others. 



