The Life-History of Penicillium
Authors: Brefeld’s Oscar , M’Nab W. R. 
Published: December 2023

Among moulds Penicillium glaucum is the one of most frequent occurrence. With unparalleled obtrusiveness the little plant forces its troublesome and unwelcome acquaintance alike on the learned and unlearned. Its pre-eminence among moulds depends less upon its size than upon its abundance, commonness, and highly characteristic pale blue colour. The fungus exists everywhere, and it is not possible by any observations to fix the limits of its geographical distribution. Its occurrence is dependent on no accident; it is the natural and necessary consequence of the ubiquitousness of its excessively minute conidia (asexually developed spores), which reproduce it with the greatest ease. The spores are scattered through the atmosphere, settling down when the air is still as a constituent of the dust ; and they are then carried to the ground by rain, &c., from whence, on becoming dry, they are again lifted and carried by the faintest atmospheric current, if the place where they were first deposited was unsuitable for their germination. Thus the fungus obtains access everywhere ; it is as unavoidable as the air by which it is carried. Out of doors it is to be found on all organic substances in a state of decomposition, more especially on the larger fungi. In our dwellings it is a real plague. Raw and prepared articles of food are exposed to its destructive influences. It moulds the cheese, the bread, fresh and preserved fruits, and many ways have been tried to destroy it or keep it under. It alone is the cause of many of our domestic arrangements, and much care must be taken during expeditions to prevent bread, flour, and other substances becoming mouldy. It is often content with the poorest food, which would be too bad for higher fungi. It lives in the human ear ; it does not shun cast-off clothes, damp boots, or dried-up ink. Sometimes it contents itself with a solution of sugar with very little inorganic matter, at other times it appears as if it preferred the purest solution of a salt with only a trace of organic matter. It will even tolerate the hurtful influence of poisonous solutions of sulphate of copper and arsenious acid. No wonder that, as this fungus is so well fitted to be everywhere victorious in the “struggle for existence,” it soon overcomes all other forms that come in its way. In the natural course of a spontaneous or artificial cultivation of mould the same tragedy is surely repeated. At first appear the statelier kinds of mould, the long-stemmed Mucorini and their allies, which fruit only on account of their more rapid growth. Among them Penicillium appears in three or four days ; at first harmless and modest, in the form of delicate white specks of mycelium. These grow with fabulous rapidity in all directions, and form large patches spreading out and covering the whole substratum. But even before this takes place there are, as a rule, noticeable in the middle of each tuft, not higher than half a line from the substratum, small, alabaster-white, thick tufts, which are the conidia-bearing aerial hyphæ of Penicillium. From the centre of the cluster, and therefore from the centre of the whole group, begins a change from white into blue, which indicates the ripening of the spores. The blue colour spreads centrifugally over the whole patch, leaving only a white edge. At length even the edges becomes blue, and in from seven to ten days the whole substratum is covered with a blue coating, which, on the slightest agitation, gives off clouds of spores. When the whole substratum is exhausted the culture of the Penicillium ends. Other fungi have no chance of growing during the period of vegetation of the Penicillium, and any subsequent development in the exhausted nutrient substance is impossible. Penicillium is the plebeian despot among the moulds—mould par excellence, and the form meant when we use the word mould.