The following definition of the term "vitamines" gives a satisfactory conception of the nature and functions of these substances, so far as they are yet known. "Vitamines; constitute a class of substances the individuals of which are necessary to the normal metabolism of certain living organisms, but which do not contribute to the mineral, nitrogen, or energy factors of the nutrition of those organisms." As sub-groups of the vitamines, there have already been recognized the growth-promoting, fat-soluble A; the antineuritic B, and the antiscorbutic C.
Until very recently, the investigations of vitamines have dealt exclusively with their relation to human nutrition; although it has been generally believed that the vitamines themselves are elaborated only by plants. It was generally recognized, however, that those plants, or parts of plants, which are capable of very rapid growth or metabolic changes, such as germs, spores, leaves, etc., are generally the richest source for vitamines for animal needs. Hence, there seemed to be considerable basis for the assumption that the elaboration of these substances by plants is definitely connected with their own metabolic needs. Recently, investigations of the functions of vitamines in the growth of plants have been begun. These are still in progress, but the following conclusions seem to be justified at the present time: (a) Potato tubers appear to contain growth-promoting substances which are essential to the proper growth of the sprouts. Whether these are the same substances which are efficient in the prevention of scurvy in men has not yet been investigated. (b) Baker's yeast is probably dependent upon a supply of vitamines in the medium in which it is to grow. Yeast itself, after having grown in barley wort, is one of the most important sources of vitamines for animal uses or for purposes of investigations of vitamine activity. But it has been reported that a yeast cell will not grow in an artificial medium which contains all the essential nutrients for yeast but has no vitamines of other plant origin in it. The addition of barley wort, containing the vitamines from barley germs, or any other similar supply of vitamines, induces rapid growth and the storage of vitamines in the growing yeast masses. (c) The growth of many bacteria is either wholly dependent upon or greatly stimulated by the presence of vitamine-like substances in the medium upon which the microorganisms grow. (d) Sclerotinia cinerea, the brown rot fungus of peaches and plums, will grow only in a medium which contains, in addition to the essential sugar, salts, and nitrogenous material, vitamines derived from either the natural host plant tissues or other plant sources. These may be of two types (namely, a vegetative factor and a reproductive factor) or two different manifestations of activity of the same vitamine substance. But both of these factors must be provided before the fungus can make its characteristic growth.
There is, as yet, no conclusive evidence on many of the matters concerning the relation of vitamines to plant growth. But it seems that these substances are of almost universal occurrence in the organic world; that they are not of the same general type as other substances which are essential to the nutrition of plants or animals, but have specific stimulating or regulating effects upon the physiological activities of the organism; that the vitamines which are essential to animal life are elaborated by plant tissues, but that in the case of the bacilli of certain human diseases there seems to be some indication that the affected tissues of the animal host produce vitamines which are essential, or favorable, to the growth of the parasitic organism. There seems, therefore, to be evidence of a mutual relation between plants and animals with respect to their nutritional needs for the so-called "vitamines." But the evidence concerning the function of these substances in the tissues of the organism which elaborates them is, as yet, inadequate to provide any clear conception of the reason for their development or of the mechanism by which they are elaborated. Neither is there, as yet, any conclusive evidence concerning the chemical nature of the substances themselves.
AUXIMONES
Certain investigations have indicated that bacteria, at least, develop exogenous vitamines which are beneficial to the growth of other plants. These are the so-called "auximones." For example, bacterized peat seems to contain auximones which may be isolated from the peat and exert a beneficial effect upon the growth of various seed-plants, including common farm crops. Neither the original experimental data, nor the theories which have been advanced to account for the observed beneficial effects of the supposed "auximones" have, as yet, sufficient confirmatory evidence definitely to establish their soundness. But it seems that there is a probability that some plants, at least, do elaborate vitamines, or auximones, which are useful to other plants.
TOXINS
Toxins are substances which affect injuriously the normal activities of the organism. As has been pointed out, they may be the same substances which, in lesser concentrations, exert a stimulating effect upon the same organism. Hence, it is probably inaccurate to discuss the toxins as a distinct group of substances.
There are, however, a large number of water-soluble chemical substances which are injurious to all living protoplasm, even at concentrations considerably less than the point of osmotic equilibrium in the juices of the protoplasm. These substances may act either directly or indirectly upon the protoplasm, but at certain concentrations they always affect it injuriously. In the main, these toxins are external agents of other than plant origin; although chemical substances developed by one plant may be toxic to other plants, or even to other organs of the same plant than those in which they are elaborated.
Toxins may be either general (i.e., injurious to all types of plants), or specific (i.e., injurious to only certain species) in their action. Examples of specific toxicity are of only minor importance in plant studies. They seem to be generally explainable on the basis of some unusual lack of resistance or failure of the susceptible plants to be able to exclude the entrance of these injurious substances into the protoplasm by "selective adsorption," or to convert the injurious substances into insoluble and non-injurious forms, as is done by other plants which are not susceptible to injury by these "specific" poisons. Hence, particular attention need not be given to this type of toxins.
Toxic substances may act injuriously upon plant tissues in a variety of ways. Many electrolytes, especially the salts of the heavy metals of high valency, coagulate protein material and the entrance of such substances into the protoplasm causes disturbances in the colloidal condition which cannot be otherwise than injurious to its normal activities. Similarly, formaldehyde and many other organic compounds may affect the colloidal properties of the protoplasmic gel in such a way as to injure the plant tissues.