338 KrirRKwooD: POLLEN-TUBE IN CUCURBITACEAE 
As to the particular organ of the embryo-sac responsible for 
the discharge of the stimulant, opinions have ‘differed, some as- 
cribing that function to the synergids and others to the egg-cell 
(Strasburger*', Lloyd"). But to the writer it seems more prob- 
able that the source of such a stimulant may be found in the en- 
dosperm-nucleus and its cytoplasm. The appearance of this 
nucleus, indicating the large degree of activity, and its abundant 
cytoplasm filled with starch would favor sucha view. The pollen- 
tube is presumably ruptured upon entering the embryo-sac by the 
rapid absorption of water and consequent increased turgor, and its 
contents discharged into the male generative nuclei come under 
the influence of the egg-cell and fertilization takes place, and in 
some cases also the fusion with the endosperm-nucleus. 
SUMMARY AND CONCLUSIONS 
1. The behavior of the pollen-tube was studied in Melothria pen- 
dula, Micrampelis lobata, and Cyclanthera explodens. As observed 
in these cases the time elapsing between pollination and the arrival of 
the pollen-tube at the embryo-sac was twenty-six hours in Melo- 
thria, nineteen hours in Micrampelis, and in Cyclanthera forty-one. 
2. The pollen-tube follows conducting-tissue which for the most 
part is formed of the epidermis lining the stylar canal and cover- 
ing the placental lobes. The tube traverses this tissue by passing 
over its surface wherever possible, and rarely affects the cells in- 
juriously. The conducting-tissue of these plants is abundantly 
supplied with starch. In Cucurbita Pepo starch is wanting in the 
conducting-tissue but present in abundance in the pollen-tube. In 
all cases observed where starch abounded in the conducting-tissue 
it was absent from the pollen-tube. The pollen-tube appears to 
be directed by nutritive substances secreted by the conducting- 
tissue. Attention is called to the similarity in anatomical features 
of the cells of the conducting-tissue to those of certain nectaries, 
as evidence as to the nature of the secretion. 
3. It is thought that the pollen-tube comes under the influence 
of a stronger stimulant emanating from the ovule. It is suggested 
that the source of this stimulus may be the endosperm-nucleus. 
4. The evidence at hand supports the contention of Miyoshi 
and others that the behavior of the pollen-tube is a phenomenon 
