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LICHENS So few of our members own to any knowledge of lichens that I am venturing to try to interest some in this great and curious division of plants. But I beg lichenologists to pass on to the next article, for there is nothing here that will be news to them. Lichens are two plants—an alga and a fungus—living for their mutual advantage in complete symbiosis with their tissues so interwoven or interlayered that they cannot be separated without destroying the plant. So completely do they combine that when Schwenderer discovered this in the mid-19th Century, it was hotly contested by the leading lichenologists of the day. A controversy arose as fierce but not as acrimonious as that of Sir Richard Owen and the Darwinians about the same time, though it did not excite the same populär interest. Nylander, Crombie, Bentham were ranged against Schwenderer. Crombie in his monograph of British Lichens in the British Museum Flora in 1894 asserted that Nylander had demonstrated that the gonidia (alga) and the hyphae (fungus) grow one from the other, and it was left for the late Miss Annie Lorain Smith (she died in 1937, aged 84) in writing the second volume to Crombie's work and re-writing his first volume in 1911 to say that an alga cell floating in the air must have got into Nylander's culture. Her Handbook of British Lichens, 1921, is a convenient abridgment of her larger work. W. A. Leighton's " Liehen Flora of Great Britain, 1879," is still a Standard work although in his preface he maintains " Lichens are a natural order of aerial cellular plants intermediate between the Algae and the Fungi." It is now possible to say without contradiction that lichens are built up of two elements : one being Chlorophyll cells like those of blue-green freshwater algae such as Nostoc [A spherical colony of Nostoc living on damp ground is sometimes called " Star-jelly " or " Witches' Butter "] ; the other being the fine tubes, hyphae, like those of a fungus mycelium. There is nothing in a liehen of the nature of wood, cork, bastfibres, annular or spiral vessels or sieve-like tubes. T h e more conspicuous species of liehen have a flat thallus attached to a tree-trunk, rock or soil by hairs or fibre-like holdfasts, known as rhizinae, but have no true roots. T h e hyphae of the fungus, stand up perpendicular to the surface of the thallus and adhere tightly together and, having something of the nature of chitin, form a very stout transparent cortex. In a few cases the cortex may be absent and the only covering may be hyphae lying on the top. Under the cortex is a layer of Chlorophyll cells, gonidia, disconnected or in Clusters or strings. In the middle is a layer or