The Role of the Micropyle in the Pollination of Pinus

1. The structure and function of the micropyle in Pinus was studied and its role in pollination described. The micropyle is shown to be a composite structure, consisting of two arms, a neck, and a canal, and is an integral part of the ovule at the time of pollination. The closing mechanism is initiated spontaneously by a stimulus from within the enlarging ovule and is not triggered by the presence of pollen germinating on the nucellus. 2. The occurrence of a secretion from the micropyle at the time of pollination was verified. This fluid was present in all receptive strobili which had been protected from stray pollen. It was most obvious in the early-morning hours and especially under conditions of high atmospheric humidity. Contact of pollen with the fluid causes permanent withdrawal of the latter, which is fairly rapid and complete. The secretion and absorption of this fluid in the absence of pollen closely resembled the phenomenon of guttation, and it is suggested that both processes are initiated in a similar fashion. 3. A chemical analysis of the micropylar fluid revealed the presence of three sugars, sucrose, glucose, and fructose, with a total concentration of approximately 1.25%. 4. From a study of the action of the micropylar fluid in transporting the pollen to the nucellus, absorption rather than flotation was found to play the dominant role.