338 Kirkwood : 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 31 , Lloyd 15 ). 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 such a 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 



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dnla y Micravipelis lobata % and Cyclanthera explode us. 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 Mclo- 

 thria y nineteen hours in Micravipelis, 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 



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that the source of this stimulus may be the endosperm-nucleus. 



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and others that the behavior of the pollen-tube is a phenomenon 



