SOME interesting problems connected with the power of vegetative reproduction arise in fungi. Notably there is the difficulty of defining the limits of the individual, and a rather arresting one that arises out of this is the development of the capacity for death. Let us consider the first problem. It is a characteristic of the group of Fungi Imperfecti that they reproduce themselves by vegetative bodies that can, if necessary, endure a surprising amount of barbarous treatment. Even in the otherwe find that the vegetative method of reproduction is retained throughout as an alternative to the
sexual method and operates when the conditions of the environment are suitable. In fact we almost think that the fungus hesitates to indulge in sexual reproduction unless some particularly unpleasant surroundings threaten the extinction of the organism. Note how death and reproduction are at once closely related even if only as alternative possibilities.
If we are examining under a microscope a confused mass of hyphrc and reproductive spores it is difficult to say where any individual fungus begins and ends. Then, when we remember that a number of apparent individuals (if we can determine their limits) can exist together at the same time in a colony, all having originated from one spore, it is obvious that the term individual may take on a new meaning. We have indeed what is known as the problem of the ‘greater individual.’ A similar occurrence is found frequently in horticultural practice where garden strains are reproduced vegetatively, for instance, from ‘‘or ‘slips.’ All the plants come from one original plant, so that all the plants may be regarded as being part of the original one. The term ‘individual ‘may thus be applied to one plant in your own garden or to all the plants in everybody’s garden.
When we are thinking of individuals in terms of trees it is sometimes possible to distinguish between the greater individual and the particular individual, but when we are dealing with a mass of microscopic fungus growth the problem becomes well-nigh impossible of solution. We can, however, look at it in this way. The term individual is one that has a fairly definite meaning when we apply it to ourselves, and it is possible to think of a number of fungi in which it would have as definite a meaning. For example, we can think of a spore of a mushroom that has been produced by a kind of sexual process, germinating and producing hyp has, making up the mass of mushroom ‘spawn ‘that the botanists call the mycelium. All the products from the one spore would make up one individual.
THE FUNGI’S EXPERIMENTS IN REPRODUCTION INDIVIDUALITY is thus some quality that becomes distinguishable only with definite sex, and in its absence we should be attempting to apply a label that cannot properly be applied at all. Our problem then is solved for practical purposes, so far as any solution of so complex a matter is
possible. When plants are reproduced vegetatively there is no such thing as an individual, and so death of an organism is not possible until individuals become differentiated as separate entities. We have become so accustomed to the idea of reproduction following sexual fusion that we regard them as cause and effect. But other organisms have other methods, and frequently these two processes are not so intimately related as they are in ourselves, and there may even be an appreciable time interval between the two. In general, it would appear that the more organisms have developed special parts of themselves to serve for special purposes, the more closely are sex and reproduction related.
In fungi almost any part can perform the function of any other part and frequently reproduction is purely vegetative. Almost any odd piece can regenerate an entire organism. Even when organisms are fully developed, however, and are made up of a number of complicated parts such as those of which the human body is made, at the moment of reproduction they concentrate all their possibilities into one cell, as it is only in a one-celled stage that the sexual fusion that precedes reproduction can take place. In fungi something even less clearly marked than this occurs. It seems almost as though, in fungi, sex itself is in an experimental phase, and that various methods are being tried.
FUNGI WITH SIXTY-FOUR SEXES
THE problem of individuality that we have discussed was shown in some respects to be only one aspect of a wider problem of reproduction. It has been insisted that fungi have two methods of reproduction; one, vegetative, the other requiring some kind of sexual fusion. Now sex and sexual processes are things that defy definition. Normally we expect a sexual process to involve some kind of fusion of two cells that we term male and female. Our confidence in this theory-is born of some acquaintance with our own bodies; it is useless, however, to take a set of ideas devised for ourselves and apply them to organisms like fungi with any hope of an accurate fit. In fungi, reproduction can, as we have seen, exist apart from sex. In as much as these two processes are not virtually one in fungi, we must enquire whether sex itself is less specialised in this group than it later becomes. We are so accustomed to two kinds of animal—male and female —between which fusion occurs, that we think of sex in terms
of the reproductive cells of two sexes. Cut conditions in fungi are so erratic that sixty-four or more kinds may exist. In one of the higher animals, when an egg-cell is ripening, small pieces of its nucleus are thrown away, so that eventually the cell becomes a concentration of protoplasmic material. A male cell on the other hand forms a number of very small cells, each one of which is little more than an active nucleus provided with a tail. The tail is a very necessary part of the organisation, because the male has to undertake an adventurous journey in order that it may reach its nemesis of an egg-cell.
Fusion of eggs and sperms rarely occurs in fungi, though what amounts to the same thing frequently happens; that is, the fusion of the nucleus of some small body with that of a larger one. Often a portion of a fungus hypha is cut off from the rest and becomes differentiated into what functions as an egg-cell and is therefore called the female part. Then quite commonly from a neighbouring portion a tube grows out and fuses with the female part. Because of this action the small tube that grows out is regarded as the male. The two nuclei fuse, so that we may be watching some kind of sexual process, though it is not the well-marked affair of the ‘higher ‘animals.
Variations on this theme occur throughout the fungi. Rarely does the male nucleus have any extensive journey to undertake outside the limits of the fungus wall, while sometimes as in rust-fungi there is a fusion of nuclei between neighbouring cells, that can hardly be regarded as, and which probably is not, sex at all. Acrimonious discussions on this particular point have raged round the nuclei of rust-fungi.