ANABÆNA LIVING IN BOTRYDIUM.—It is now well known that many plants belonging to the group of the Nostocs flourish within the cells of other plants. Thus they are to be found in the petioles of the leaves of Gunnera, in Lemna, in Anthoceros, in Blasia, and in Azolla; and it was to be expected that they would equally find themselves at home in the cells of even more lowly organised plants. An instance of this latter, not without interest, has been noticed by Dr. L. Marchand, who recently collected a Botrydium at Montmorency, which, on being examined under the microscope, was found, instead of containing the usual mass of granular chlorophyll, to be filled with a chain of moniliform filaments, presenting all the characters of the chaplets of a Nostoc or Anabæena. These filaments were composed of cells, some oblong with yellowish heterocysts, and they did not fill the entire cavity of the Botrydium cell, but seemed to adhere to its inner walls. The Botrydia plants were perfect; the root-like prolongations, as well as the rest of the plant, were perfectly closed. Plow then did these foreign bodies get in? This is not a question easy to answer, but it is one well worthy of being investigated. Dr. Marchand calls attention to the remarkable figure of Mr. K. Parfitt in “Grevillea” (vol. i. p. 103, pi. vii.), in which there is now little doubt, with the light thrown on the subject by Dr. Marchand's specimens, that there is represented our common species of Botrydium with a parasitic, or better, an endophytic Anabæna. No doubt the cells of the Anabæna in Parfitt's figure are badly represented, but the observation made in Parfitt's paper would seem now not to be without a special interest of its own.