Chapter VIII: Conclusions

In the foregoing chapters a very limited number of plant poisons have been considered, yet there is sufficient evidence to show that even these few differ considerably in their action upon plant-life. This action is most variable, and it is impossible to foretell the effect of any substance upon vegetative growth without experiments. The degree of toxicity of the different poisons is not the same, and also one and the same poison varies in the intensity and nature of its action on different species of plants. While certain compounds of copper, zinc and arsenic are exceedingly poisonous, compounds of manganese and boron are far less deleterious, so that a plant can withstand the presence of far more of the latter substances than of the former. Again, the tested compounds of copper, zinc and arsenic do not seem to stimulate growth, even when they are applied in the smallest quantities, whereas very dilute solutions of manganese and boron compounds decidedly increase growth. But, differentiation occurs even in this stimulative action, for while manganese is the more effective in stimulating barley, boric acid is far more potent for peas, the shoots being particularly improved.

A consideration of the experimental work that has been done on this subject of poisoning and stimulation leads one to the inevitable conclusion that it is not true to maintain the hypothesis that _all_ inorganic plant poisons act as stimulants when they are present in very small quantities, for while some poisons do increase plant growth under such conditions, others fail to do so in any circumstances. It is probable that what has been found true with the few substances tested would prove to be similarly true over a much wider range of poisons, and at any rate the hypothesis must be dismissed in its universal application. A more accurate statement would be that some inorganic poisons act as stimulants when present in small amounts, the stimulating concentrations varying both with the poisons used and the plants on which they act.

It is quite possible for a stimulation in one respect to be correlated with a retardation in another. In the Rothamsted experiments on the action of manganese sulphate on barley the weaker concentrations of the salt improved the vegetative growth, as was shown by the increase in the dry weights, but with the same strengths of the poison the ripening of the grain was retarded, so that, while certain of the physiological functions were expedited, others were hindered by the action of the poison.

Thus it is evident that it is exceedingly difficult sharply to characterise either toxic or stimulant action. In neither case is the reaction simple--many factors may come into play and many processes are concerned, while the effect of a so-called poison may vary in respect of each of the functions and processes concerned. If the poison is presented in great strength the toxic action is dominant, and probably affects many functions in the same sense, so that the action is, so to speak, cumulative. Lower down in the scale of concentration differentiation of action may set in, and while some processes may still be hindered, others may be stimulated. If the two actions balance one another an apparent indifference may be manifested, so that it seems that such strengths of the poison have no effect on growth, either harmful or beneficial. At still lower concentrations, with certain plants and certain poisons, the stimulative action overpowers the toxic effect, so that in some respect or other improvement occurs in growth.

It is quite conceivable, however, that some poisons are truly indifferent in weak concentrations, as no stimulation makes itself evident under any circumstances. In these cases one is inclined to suspect that the action is somewhat more simple, in that the toxic effects gradually diminish until no poisonous action is manifest at very weak concentrations, and as no stimulation is present to bring the growth above the normal with these very weak concentrations the plant is similar to those grown without any addition of the poison.

The _modus operandi_ of these stimulative agents is not yet fully understood. Perhaps at the present time two main theories hold the field: (1) that they act as catalytic agents, being valueless on their own account, but valuable in that they aid in the procuring of essential food substances; (2) that the stimulants themselves are of integral value for nutrition. The French school, with Bertrand at the head, hold strongly to the catalytic theory, maintaining that manganese and boron compounds are able to increase growth if they are present in small quantities, as they act as “carriers” whereby the various functions of the plant are expedited by the increased facility with which the essential nutritive elements are supplied. The manganese in laccase, for instance, is held to be an oxygen carrier, whereby the oxygen is first absorbed and then released for the benefit of the plant, the manganese being regarded as essential for the functioning of the enzyme. But, if these elements are _essential_, this theory seems to stop short of the truth. If certain functions are dependent for their very occurrence upon the presence of even minute traces of any element, then surely that element is as essentially a nutrient element, as vital to the well-being of the plant as is such an element as carbon or nitrogen or phosphorus, even though the latter occurs in far greater quantity. It is necessary that one should free one’s mind from the idea that the quantity of an element present in a plant is an index of its value to the plant. Naturally enough, in the early days of plant physiology, the most abundant elements first engaged the attention of investigators, and they were divided into essential and non-essential, ten elements being classed in the former category. More recent work is beginning to show that other elements are constantly present in plants, but in such small quantities that the older and cruder methods of analysis failed to reveal them, so that until lately they have been completely overlooked in work on plant nutrition. Even yet we do not know which of these other elements are essential and which are merely accidental. While we do know that the ten essential elements (C, H, O, N, S, P, K, Mg, Fe, Ca) are necessary for the well-being of all plants, it is conceivable that these other substances which only occur in very small quantities may be more individual in their action, and that while a trace of a certain element may be absolutely essential to one plant, that same element may be quite indifferent for another species. If one takes a broad outlook, the two theories seem to be in reality only parts of one, the “nutrition” theory carrying matters a little farther than the “catalytic” idea, broadening its scope and extending its application.

It seems probable that all the experimental work that has been discussed will prove to be simply preliminary to a far greater practical application of the principle of stimulation or increased growth. While the physiologists have been feeling their way towards the conclusions put forth on this subject, the agriculturists have been discovering and extending the application of artificial manures, until at the present time such manuring is coming into its own and is receiving more of the widespread attention that it deserves. The possibility now exists that in some respects the two lines of work are converging and that the more purely scientific line will have a big contribution to make to the strictly practical line. Artificial manuring aims at improvement of the soil and crop by the addition of food substances that are needed in a particular soil, a result that used to be obtainable only by the use of the bulky farmyard manure, seaweed, &c. Apart from any other aspect of the matter the artificials, when intelligently used, are far more easy to handle and to regulate in supply, and they yield excellent results, especially in conjunction with a certain proportion of organic manures. The further prospect now opened up is the possibility of utilising some of these stimulating compounds as artificial manures. As only small traces are beneficial, larger amounts being poisonous, it is obvious that only small quantities would be needed, and, as the compounds are not usually very expensive, a considerable increase of crop for a relatively small outlay might be anticipated if no complicating factors intervened. Very much work will be required in the field to test the value of these substances, as their action may be influenced by the nature of the soil, climatic conditions, general conditions of manuring, and the crops grown. Some tests have already been made, especially in Japan, with boron and manganese, and these indicate a promising field for investigation.

Above all, it is most important to realise that one is approaching an entirely unexplored field, and that it is inevitable that the results of the initial experiments will be contradictory, at least in appearance, so that it is necessary to keep an open mind on the subject, being ready to modify one’s ideas as circumstances require, as improved experimental methods lead on to more accurate results.

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INDEX OF PLANT-NAMES

_The symbols after the plant-names represent the elements referred to on the pages indicated._

Acorus Calamus
Mn, 79

Actinostrobus pyramidalis
Mn, 80

Aesculus
Mn, 78

Agathis robusta
Mn, 80

Ageratum
Cu, 24;
Zn, 42

Alder
As, 53

Algae
As, 62, 64;
B, 66, 77

Allium
(_see_ Onion)
Zn, 47

Alnus incana
Mn, 79

Aloe
Cu, 26

Amomum sp. (Paradieskörner)
Cu, 15

Ampelopsis
Cu, 32;
Zn, 47

Anaboena
Cu, 35

Angiosperms
B, 66

Anthracnose
B, 76

Apple
B, 65, 66

Apricot
Cu, 16

Araucaria Bidwilli
Mn, 80
Cunninghamii
Mn, 80

Armeria sp.
Zn, 38
vulgaris
Zn, 36, 37

Arundo Sacchar
Mn, 78

Asparagus
Mn, 79

Aspergillus
Cu, 33, 34

Aspergillus niger (= Sterigmatocystis nigra)
Zn, 48, 49, 50;
As, 63;
B, 76;
Mn, 90

Athrotaxis selaginoides
Mn, 80

Barley, 11, 13;
Sr, 5;
Cu, 16, 17, 20, 29;
Zn, 37, 39, 40, 44, 46;
As, 52, 54, 55, 57, 60, 61;
B, 66, 69, 75;
Mn, 79, 81, 82, 83, 84, 85, 86, 89, 90, 92, 93, 94

Beans
Cu, 16, 17, 26;
Zn, 47

Beech
Mn, 79

Beetroot (Beta vulgaris)
Cu, 16, 26;
Zn, 37, 39;
Mn, 78, 79

Beet, sugar
B, 65

Betula alba
B, 66

Birch
Mn, 79

Botrytis cinerea
Mn, 90

Brassica oleracea
Mn, 78

Buckwheat, 11;
Cu, 16, 17, 29, 30;
As, 53
(_see_ Polygonum Fagopyrum)

Cabbage
Sr, 5;
As, 51, 52;
Mn, 87

Cacao
Cu, 16

Callitris gracilis
Mn, 80
robusta
Mn, 80

Cannabis
B, 72
sativa
B, 66

Capsella Bursa-pastoris
Zn, 37

Cardamomum minus
Cu, 15

Carrot
Mn, 79

Cauliflower
Mn, 78

Cherry
B, 65

Chestnut
Ca, 71;
Mn, 79

Chickpea
Cu, 16;
B, 66

Chicory
Mn, 79

Chrysanthemum
B, 66

Clover
Zn, 42, 44

Colea
Cu, 24;
Zn, 42;
As, 58

Collinsia
B, 72

Coniferae
Zn, 38

Conium maculatum
Mn, 78

Colza
B, 74

Couch grass
Mn, 83

Cow pea
Cu, 18

Cruciferae
Mn, 78, 89

Cryptomeria japonica
Mn, 88

Curcuma longa
Cu, 15

Currant
Cu, 31

Dacrydium Franklini
Mn, 80

Dianthus caryophyllus
Mn, 79

Elodea canadensis
Cu, 32;
Zn, 48

Fagopyrum esculentum
Cu, 29

Ferns
B, 66

Fig
Zn, 42;
B, 65, 66

Fir
Mn, 80

Flax
Mn, 78, 88, 89

Fungi
Cu, 33;
Zn, 44, 50;
As, 64;
B, 66, 77

Geranium
Cu, 24;
Zn, 42;
As, 58

Gramineae
B, 72;
Mn, 89

Grasses
Zn, 39, 40, 42

Gymnosperms
B, 66;
Mn, 80

Haricot
B, 71, 72
green
As, 52
white
Zn, 37

Hemp
Sr, 5

Hop
B, 66

Hordeum distichum
As, 54
vulgare
Zn, 39, 47
(_see_ Barley)

Iberis
B, 72

Laminaria saccharina
B, 66

Leguminosae
B, 72;
Mn, 78, 89

Lentil
Mn, 79

Lichen
As, 59

Linseed
Cu, 16, 17

Linum
B, 72

Love-apple
B, 66

Lucerne
Mn, 79, 91

Lupin
Cu, 17, 19;
As, 59
white
As, 61;
B, 67, 70
(_see_ Lupinus albus)
yellow
B, 70, 75

Lupinus albus
Cu, 19, 20, 22;
Zn, 45;
B, 68
(_see_ White Lupin)

Maasa picta
B, 65

Maize
Cu, 16, 17, 19, 24, 26, 27;
Zn, 37, 44;
As, 53, 54, 60;
B, 67, 68, 71, 72, 74;
Mn, 79, 81

Menyanthes trifoliata
Cu, 35

Mildew
B, 76

Molinia cærulea
Cu, 16;
Zn, 37

Mould
B, 76

Mucor mucedo
As, 59, 63

Mushroom
B, 66

Mustard
Cu, 17

Nasturtium
Cu, 17

Nuphar lutea
Cu, 35

Oak
Cu, 16;
Zn, 42

Oat
Cu, 16, 17, 19;
As, 53;
B, 74;
Mn, 79, 82, 86, 87

Onion
B, 66;
Mn, 88

Onobrychis sativa
Zn, 39

Opium
Mn, 79

Oscillatoria
Cu, 35

Panicum italicum
Cu, 26;
As, 58

Pansy
Cu, 24;
Zn, 42;
As, 58

Paprika
Cu, 16, 17

Paradieskörner (Amomum sp.)
Cu, 15

Pea
(_see_ Pisum sativum)
sweet
Cu, 17

Pear
Cu, 16;
B, 65

Penicillium
Cu, 33, 34
brevicaule
As, 63
cupricum
Cu, 34
glaucum
Cu, 23;
As, 59, 63;
Mn, 90

Phaseolus vulgaris
Cu, 17;
As, 59

Phormidium Valderianum
As, 62

Phyllocladus rhomboidalis
Mn, 80

Pine
Mn, 80

Pineapple
Mn, 83

Pisum arvense
Cu, 29
sativum
Cu, 17, 18, 26, 27, 29;
Zn, 41, 47;
As, 58
(“Pea”), 3, 11, 13, 93;
Cu, 17, 26, 29, 30;
Zn, 40, 46;
As, 51, 54, 55, 56, 58, 61;
B, 67, 73, 74, 75, 93;
Mn, 79, 81, 82, 85, 86, 87, 89, 92

Plantago lanceolata
Zn, 37

Podocarpus elata
Mn, 80

Polygonum amphibium
Cu, 35
aviculare
Zn, 37, 38
Fagopyrum
Cu, 26, 39;
Zn, 41;
As, 54, 58
(_see_ Buckwheat)
Persecaria
Cu, 5;
As, 54

Poplar
Cu, 15

Potato
Cu, 16, 26, 27, 30;
As, 52;
Mn, 78, 79

Protococcus infusionum
As, 62
sp.
As, 63

Pyrus communis
Mn, 79

Radish
Sr, 5;
B, 74;
Mn, 84, 90

Raphanus
B, 72
sativus
Zn, 39

Raspberry
As, 65

Rice
Zn, 47;
B, 66, 73;
Mn, 79, 86, 87, 88

Rosa remontana
Mn, 79

Rubus
B, 66

Rye
Cu, 16;
As, 60, 61

Sainfoin
Mn, 79

Secale cereale
Cu, 26;
Zn, 41;
As, 58

Silene inflata
Zn, 36, 37

Solanum tuberosum
Mn, 78

Soy beans
Cu, 17, 19;
B, 67;
Mn, 81, 85

Spinach, 41;
B, 73;
Mn, 89

Spergula arvensis
Zn, 39

Spirogyra
Cu, 35;
As, 62;
B, 76

Stichococcus bacillaris
As, 62

Sterigmatocystis nigra
Zn, 48, 49
(_see_ Aspergillus niger)

Sugar cane
B, 65

Taraxacum officinale
Zn, 37

Tea
Mn, 79

Thlaspi alpestre
Zn, 36
sp.
Zn, 38

Tobacco
B, 66

Trapa natans
Mn, 79

Trifolium pratense
Zn, 39

Triticum vulgare
Cu, 17;
B, 67
(_see_ Wheat)

Tropeolum Lobbianum
Cu, 17, 18

Turnip
As, 51, 52;
B, 74
swede
Mn, 78

Tussilago Farfara
Zn, 37, 38

Ulothrix tenerrima
As, 62

Ustilago
Cu, 28

Vaucheria
B, 76

Vicia Faba
Sr, 5;
Cu, 27, 29
sativa
B, 67;
Zn, 39

Viola sp.
Zn, 38
tricolor
Zn, 36

Vine
Cu, 31

Vitis vinifera
As, 52

Watercress
B, 66

Water-melon
B, 65

Wheat
Cu, 16, 17, 23;
Zn, 37, 44, 46;
As, 52, 60;
B, 66, 70, 72, 73;
Mn, 79, 81, 83, 84, 85, 86, 87

Willow
Zn, 39, 40

Yeast
Zn, 50;
B, 76

Zea japonica
Cu, 17, 18
Mays
(_see_ Maize)

GENERAL INDEX

Absorption capacity of soils for zinc, 41
of poisons by plants, 25

Accelerators, 45

Action of heavy metals in mixed solutions, 20

Adsorption, 8, 23

Aeration in water cultures, 8

Algae, assimilation of arsenic by, 62
clearing ponds of, 35
effect of arsenic on, 62
effect of boron on, 76
effect of copper on, 35

Aluminium, 45, 78

Arbutin, 4

Arsenate, potassium, 53, 62, 63
sodium, 55, 57, 61

Arsenates, 53, 57

Arsenic acid, 53, 54, 60, 61, 62, 64
acid v. arsenious acid, 53
acid v. phosphoric acid, 53, 62
elimination of, 59
gas liberated by moulds, 63
in soil, effect of, 58
in superphosphate, 58

Arsenious acid, 53, 54, 57–61, 64
immunity of plants to, 58

Arsenite, sodium, 55, 56, 61

Arsenites, 53, 57
v. arsenates, 57

Artificial oxydases, 84
soil, 24, 46

Assimilation, reduction in water plants, 48

Barium, 44

Borate, calcium, 71
potassium, 71

Borates, 72

Borax, 71, 73

Bordeaux mixture, 30
blocking of stomata by, 33
on assimilation, effect of, 33

Boric acid, 1, 65–76, 93

Boromannitic acid, 68

Boron, antiseptic action of, 70
colour due to, 75
distribution in plants, 66
poisoning, indication of, 68, 69
rôle in plant economy, 74

Cadmium, 31

Calamine, 37
plants, 38
soils, flora of, 37

Calcium carbonate, 4, 23, 25
chloride, 20, 59
sulphate, 20, 44, 45

Carbon black, 23
dust, 10

Catalytic elements, 49, 91

Chlorophyll, 44, 60

Complementary manures, 47, 91

Conditions of plant life, 7

Copper, acetate, 19
action on plant organs, 30
bicarbonate, 26
bromide, 19
chloride, 19, 20
compounds, corrosive action on plant roots, 5, 27
distribution in tissues, 16
mode of action on plants, 25
nitrate, 19, 25
oxide, 15, 25
quantity in certain plants, 17
salts, injection into plant tissue, 31
sprays, effect on leaves, 30, 32
sulphate, 5, 19, 20, 22–27, 29–35, 41

Cumarin, 4

Distilled water, preparation of, 10

Double decomposition in soil, 25

Duration of experiments, 13

Experimental methods, comparison of, 14

Ferric chloride, sublimed, 86
hydrate, 23

Fungi, effect of arsenic on, 63
effect of boron on, 76
effect of copper on, 33, 34
effect of manganese on, 90
effect of zinc on, 48

Galactose, 76

Glucose, 76

Germination, effect of arsenic on, 60
effect of boron on, 72
effect of copper on, 27, 28
effect of manganese on, 84
effect of zinc on, 43
of seeds in sawdust, 11

Grading of seeds, 11

Growth in copper-distilled water, 17
of peas in water cultures, 11

Hydrochloric acid, 22

Hydroquinone, 91

Hypothesis of universal stimulation, 93

Iodine, 2, 91

Individuality of plants, error due to, 13
of species, 61

Interaction between soil and poison, 9

Iron, 31, 49
oxide, 78
sesquioxide, 84
sulphate, 48, 81, 85, 88

Laccase, 91, 95

Lack of control over field experiments, 9

Lead, 10, 26, 42, 44

Magnesium carbonate, 83, 84, 87
chloride, 20
sulphate, 1

Manganese as top-dressing, 89
chloride, 86, 88
commercial value of, 88
cytological action of, 81, 82
dioxide, 84, 87
essential to Coniferae, 80
in Australian soils, 83
in leaves, deposition of, 82
in organic combination, 79
iodide, 83, 84
manuring, after-effects of, 88
nitrate, 83, 84
oxide, 78, 79, 83, 84, 86, 88
phosphate, 79, 83, 86
retardation of ripening by, 82, 85
sulphate, 81–89, 94

Masking effect of inorganic food salts, 4, 20

Mercuric chloride, 22
oxide, 41

Mercury, 26

Metallic oxides, 78

Methods;
field experiments, 9
sand cultures, 8
soil cultures in pots, 9
water cultures, 7, 11

Mode of entry of poisons into plants, 4

Nickel, 24, 50

Nucleinic combination, 79

Nutrient solutions, composition, 13

Oligodynamic action, 28

Over-mineralisation of plants, 71

Phosphoric acid, 53, 54, 62, 64

Photo-synthesis, effect of copper on, 32

Potassium hydrate, 22

Presence of arsenic in animals, 51
in plants, 51
of boron in plants, 65
of copper in plants, 15
of manganese in plants, 78
of zinc in plants, 36

Pyrogallol, 91

Raulin’s solution, 49

Reproduction of poisoned plants, 40

Silver nitrate, 22

Sodium chloride, 20, 44
hydroxide, 22
nitrate, 4

Sterile cultures, 24

Stimulation, by injection of copper solutions, 31
by small doses of poisons, 2
definition of, 2
local, 47
of Aspergillus niger, 50
of fungi by copper, 34
of plants by arsenic, 61
of plants by boron, 73
of plants by copper (negative), 28
of plants by manganese, 84
of plants by zinc, 45–47
physiological considerations of manganese, 90

Strontium sulphate, 5

Sugar, 22, 31, 48, 49, 50, 68

Sulphur, flowers of, 10

Thymol, 22

Toxic action, effect of arsenic, 52
effect of boron, 67
effect of copper, 17
effect of light on, 44
effect of manganese, 81
effect of zinc, 38
equivalent, 18
limits of plant poisons, estimation of, 26

Toxicity, of nutrient salts, 1
of organic compounds, 4
of poisons, cause of, 22
of positive ions in copper compounds, 19, 22
reduction of, 39, 44
reduction of, by carbon black and ferric hydrate, 23
reduction of, by insoluble substances, 22

Toxin and nutrient, distinction between, 3

Transmission of power of resistance, 72

Valency, effect on toxicity, 44

Vanillin, 4

Variation in results on different substrata, 24

Zinc, absorption by roots, 42
carbonate, 38, 42, 43
effect of, on lower plants, 48
effect of, on plant and soil, 41
fixation of, 49
mode of action on plants, 43
oxide, 37, 47
oxide on leaves, deposition of, 47
storage in seeds, 43
sulphate, 38–49
sulphide, 42

CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS.

CAMBRIDGE AGRICULTURAL MONOGRAPHS

GENERAL EDITORS: T. B. WOOD, M.A., Draper’s Professor of
Agriculture in the University of Cambridge, and E. J. RUSSELL,
D.Sc., Director of the Rothamsted Experimental Station,
Harpenden, Herts.

Each volume of this series will contain a summary of the present position on some particular aspect or branch of agricultural science by an expert of acknowledged authority.

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~The Strength of Wheat Flour.~ By Professor T. B. WOOD, M.A.

~The Constitution of the Soil.~ By E. J. RUSSELL, D.Sc.

~Disease Resistance.~ By R. H. BIFFEN, M.A., F.R.S., Professor of
Agricultural Botany in the University of Cambridge.

~Poisonous Plants.~ By H. C. LONG.

THE CAMBRIDGE FARM INSTITUTE SERIES

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~Plant Life in Farm and Garden.~ By Professor R. H. BIFFEN, M.A.

~The Feeding of Farm Animals.~ By Professor T. B. WOOD, M.A.

~A Student’s Book about Soils and Fertilizers.~ By E. J. RUSSELL,
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~Common Fungus and Insect Foes.~ By F. R. PETHERBRIDGE, M.A.

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