No cell, either a bacterial organism or any other, can grow in size indefinitely, because physical difficulties of nutrition cause the inner portion of the cell and the actual wall itself

to increase disproportionately. In bacteria this limit is quickly attained under favourable growth conditions, and then the cell divides into two. It is said that bacteria divide directly without a nuclear division, but other authorities dispute this and it may well be that eventually a normal nuclear division will be demonstrated for these organisms. A division may take place every twenty or thirty minutes.

It can be shown that a mass of bacteria increases in size according to the compound-interest law in the same way as a sum of money increases. Estimates based on cells dividing once an hour show that in two days the descendants of one cell would number 281,500 millions. Lack of food and the accumulation of waste-products prevent this rapid growth, and the cells will then die according to the operation of the same law. If bacterial growth is being held in check by the application of a disinfectant it is thus possible to say how many will die in a certain time.

Occasionally bacteria may produce spores instead of the normal vegetative cell and then the thick wall of the spore

will enable the organism to endure conditions that would otherwise be fatal. The spores, for example, of the anthrax bacillus, a germ that causes a disease in sheep and cattle and at times also in man, are highly resistant to adverse conditions and may remain quiescent but alive in the soil for many years.

The shapes of bacteria are important when distinguishing one from another, but since the investigation of bacteria has nearly always been made from some practical point of view, we distinguish them generally by the part they play in the world. We group them according to their effects, and so speak of the nitrogen-fixing bacteria which bring the free nitrogen of the air into combination with other elements; and group together the kinds that produce heat, such as those that cause hay-ricks and bales of cotton to take fire; those that produce phosphorescence on decaying fish, and so on.

The fungi form four main groups, distinguished according to the way their reproductive portions are built. All fungi may be said to consist of two portions, a vegetative part that is concerned with the absorption and assimilation of food and a reproductive portion containing a number of reproductive units, called spores.

When a spore germinates it pushes out a small tube which grows and becomes a hypha. A number of threads of hyphae may make a felt-like mass that is vegetative and is known in the aggregate as the mycelium. All the bizarre shapes of fungi, from the ordinary mushroom shape, to puff-balls, truffles and the mildew that grows on bread and cheese are made up of compacted masses of hyphae. The vegetative part of the mushroom is the mycelium or spawn that runs underground. The mushroom itself is the part which bears the spores. They are to be found in countless numbers in the ripe mushroom, borne on the radiating gills under the cap. The fact that no matter how the shape of the mushrooms or toadstools may vary, the spores are borne in groups of four, serves to distinguish mushrooms and their allies from other groups of fungi.

For example, although completely unlike in appearance and possessing no massive toadstool-like spore-bearing body, the rusts parasitic on wheat and other cereals also bear their spores in groups of four and are therefore regarded as belonging to the same general alliance as the mushroom.

An alternative arrangement that is characteristic for another of these great groups is that the spores, instead of being on

a free surface, are enclosed at first in small bags that tie the spores together in groups—usually groups of eight. Ultimately the bags burst and the spores are exploded into the air. Here again the range of variation of size and shape is considerable, with more or less massive plants like morels at one end of the scale and microscopic plants like yeasts at the other. Both of these two groups have a number of transverse walls at intervals in their hyphal threads and this fact serves to distinguish them from the next big group that is usually without transverse walls. This group includes the moulds such as the common one that grows on moist bread. From the cobwebby mass that grows over the surface erect threads arise that cut off spores in special receptacles. Various relatives of this mould are of special interest. There is one, for example, that grows on oil cake and fodder, while another is the well-known ‘fry-cholera,’ a disease by means of which unsuccess-

ful attempts have been made from time to time to provide a biological control of the house-fly. A number of aquatic fungi are also allies. The devastating disease of salmon and other fresh-water fish belongs to this group as also does the fungus that kills seedlings of plants that have been allowed to develop in too much moisture. Of increasing importance are some aquatic forms that are parasites of pond-weeds and of microscopic plants and animals that form part of the food of fishes.

FUNGI THAT LEAD SECRET AND DESTRUCTIVE LIVES THE fourth great group of fungi is a remarkable collection of odds and ends, which are alike only in the facts that little is known about them and that they often make themselves great nuisances. This group is known as the Fungi itnperfecti, and the word ‘imperfect ‘implies rather that their life histories are extremely simple or that they appear simple because of our lack of knowledge of them. For example in many of the ‘imperfect fungi ‘no reproductive phase is known. The reproductive phase has either been omitted or has been lost or else remains still to be discovered.

A large number of these ‘imperfect fungi ‘cross man’s path at various points, and because of the economic importance they have assumed they have been subjected to detailed examination, with the result that some have been shown to be stages in the life-history of ‘perfect ‘forms. It is preferable to use the term ‘perfect ‘stage rather than to speak of a sexual stage, for, as will be shown later, the problem of sex in fungi is so complex that the subject is even more difficult than it is in human life.

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