'Before Dr Dillenius gave me a hint of it, I took no particular notice of mosses, but looked upon them as a cow looks at a pair of new barn doors.'
The celebrated American botanist John Bartram wrote this in the eighteenth century; to-day, though much has been written about mosses since Dr John James Dillenius published his bulky Historia Muscorum in 1741, most people, even many professed botanists, would probably admit that they had taken ‘no particular notice’ of mosses. Yet mosses can be studied anywhere and at any time and they are among the most beautiful of plants, though much of their beauty can be seen only with the help of microscope or hand-lens. According to some, ‘the hyssop that springeth out of the wall’ which King Solomon studied was a moss.
THE LIFE-HISTORY OF A MOSS
The story is thus a remarkable one, the spore growing first into the thread-like protonema, then into something seemingly quite different, the leafy moss plant which reproduces sexually to give something quite different again, the stalked spore capsule. In the technical language of botanists the moss is said to have two ‘generations’ in its life-history, the leafy moss plant (with its youthful stage) and the leafless spore capsule with its slender stalk. The latter does not become an independent plant, but remains attached to the parent leafy shoot, on which it is partly parasitic since it depends on it for a large part of its nourishment. The moss is thus said to show ‘alteration of generations’ and the two generations are quite unlike both in their appearance and their method of reproduction.
A tuft may develop from a single spore or from several which have germinated close together but in either case it may be very difficult to trace a connection between the stems in a tuft, so that to say exactly what constitutes an individual moss plant is by no means easy. In many mosses the stem has the power of indefinite growth and rots away only slowly at the base, so that the tufts may reach prodigious size. Dr Spruce wrote of Leucobrycum glaucum at Strensall Moor, near York: ‘…It forms immense rounded socks, some of which in my youth were as much as three feet high… When the late Mr Wilson (a celebrated student of mosses) first saw them, he took them at a distance for sheep, as he approached them he changed his mind for haycocks; but when he actually came up and saw what they were he was astonished and declared he had never seen such gigantic moss-tufts elsewhere.’
Few microscopic objects are more fascinating to study than the peristome teeth which guard the mouth of the spore capsule in mosses. One of their most remarkable features is their sensitiveness to slight changes in the humidity of the air. They bend inwards in moist air and outwards in dry, or, in some kinds of moss, inwards in dry air and outwards in moist. These movements are rapid and can easily be observed by breathing gently on an open capsule and watching it under a magnifying glass. In a few species, e.g. Atrichum undulatum (Plates 1 (a) and 2), the teeth, which are differently constructed from those of other mosses, do not move. Plate I will convey a faint idea of the variety of form and intricacy of construction of moss peristomes. Sometimes there is the ring of teeth, sometimes two, an outer and an inner. When there are two rings it is only the outer teeth which bend in response to changes of humidity. The number of teeth in each ring may be four, sixteen, or sixty-four; it is always a multiple of four. The individual teeth of the outer peristome are elaborately ornamented with cross-bars and ridges; under the high power of the microscope they often show a finely sculptured pattern as well. In many cases the tooth is forked towards its tip or perforated by small holes. The colour is generally yellow, reddish brown, or bright crimson. The inner peristome, when present, is more delicate and fragile than the outer and is usually light in colour.
JOHANNES HEDWIG AND HIS BOOKS ON MOSSES
The plates with which this volume is illustrated are taken from Johannes Hedwig’s Descriptio et adumbratio microscopico-analytica muscorum frondorsorum (Leipzig, 1787-97). This book, because of the accuracy of both text and illustrations, was a landmark in the study of mosses; its author was one of the most remarkable of the many great figures in late eighteenth-century botany and one of the best microscopists of his age, so a few notes on him and his work may be of interest.
Johannes Hedwig was born in 1730 in the beautifully situated town of Kronstadt (or Brasso) in Siebenburgen (Transylvania), part of the Carpathians which has in recent times sometimes deemed part of Hungary and sometimes of Rumania, but in the eighteenth century was part of the Austro-Hungarian Empire. Like most botanists of his day, Hedwig was by profession a medical man. He worked first at Chemnitz and later at Leipzig, where he obtained an appointment in 1784 at the Military Hospital and in 1786 became professor Extraordinary of Medicine. His main love, however, was for botanical research, which he had long pursued, and in 1789 he was appointed Professor of Botany in Leipzig. Hedwig’s work covered several branches of botany, but he is remembered chiefly for his researches in plant anatomy and the mosses. His books on mosses were a great improvement on anything that had been done previously because of the accurate description and delineation of microscopic detail. The excellence of Hedwig’s work was partly due to the technical improvements in microscopes which had been made during the eighteenth century, and to his skill and thoroughness in observation.