KIRKWOOD: POLLEN-TUBE IN CUCURBITACEAE S34 
tions are reversed, and starch is entirely absent from the ample con- 
ducting-tissue, thoughabundant in the pollen-tube almost tothe 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 inthe tube. It is, 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 Digita/is and numerous 
other plants show that their tubes are sensitive to the presence 
of certain soluble carbohydrates, but are indifferent to or repelled 
bya 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 thatthe 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. 
