WILL you allow me to ask Mr. Hubert Airy, in reference to his interesting paper on the â Microscopic Examination of Air,â in NATURE, vol. ix. p. 439, on what ground he refers the â triangular pollenâ captured on his slide to the birch and hazel? Observations of my own have led me to the conclusion that the pollen of plants which depend exclusively on the wind for their fertilisation is perfectly spherical, at all events before the form of the grain is disturbed by the emission of the pollen-tubes, and this indeed one might expect from à priori considerations. Among the pollen-grains I have especially observed, are those of Corylus avellana, Betula alba, and Populus balsamifera. I shall be much obliged if any of your readers could refer me to any accurate published description of the form of pollen-grains beyond those contained in Fritzsche's â Beiträge zur Kenntniss des Pollen.â