Journal of the Royal Horticultural Society. 29: 77-81 (1904-5)
THE HEREDITY OF ACQUIRED CHARACTERS IN PLANTS
By REV. PROFESSOR G. HENSLOW, M.A., V.M.H., &c.
WHEN Darwin propounded his theory of Evolution, or "the origin of species by means of natural selection," he assumed two things as axioms, viz. variability, i.e. the power in plants and animals to vary, and the heredity of acquired variations.
He was not so much concerned with the causes of variations in animals and plants; but, given the variations, natural selection would, he maintained, decide which was the best fitted to survive in the struggle for life. That changed external conditions of life are somehow the primary cause of variations is admitted by all; and we must look to the living Protoplasm, together with its nucleus, as the instruments in organisms, which can be affected and excited by external influences, to set up new variations in their structure, such being acquired characters.
|* By "habit," it is usually meant that the duration of life, e.g. whether annual, biennial, or perennial, or a dwarf size, a procumbent position, &c., is characteristic.
+ Origin of Species, 6th ed. p. 10. Ibid. p. 67.
§ I am not here concerned with Weismann's theory of its immortality.
These consist of any changes in the outward form or internal anatomy of structures, as well as the "habit"* of a plant induced by external agencies.
With regard to heredity Darwin's words are: "The correct way of viewing the whole subject would be to look at the inheritance of any character whatever as the rule, and non-inheritance as the anomaly."+ Elsewhere he says: "Natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations profitable at that age, and by their inheritance at a corresponding age."
Dr. Weismann, however, introduced the word soma (i.e. "body ") for the vegetative organs of plants, viz. roots, stems, leaves, and the external organs of the flower would be included; but, on the other hand, he distinguished an imaginary "germ-plasm"§ in the reproductive organs, such as the embryo-sac, as distinct from the general protoplasm or living substance everywhere else within all living cells of the plant's soma.
It was this hypothetical substance to which alone he attributed, by means of its continuity, the power of carrying on all hereditary characteristics in the offspring.
When his attention was called by botanists to the fact that certain plants, such as Begonias and Mosses, are very generally propagated by means of fragments of their leaves &c., i.e. his "soma," he supposed that some portions of the germ-plasm must have been diffused with the protoplasm in such cases.
Since, however, the propagation of plants by their vegetative organs of any kind is possible and frequently done, both in nature and artificially, the probability of germ-plasm being always associated with protoplasm would seem to form a necessary part of the theory.
But, then, the converse becomes at once more probably true, viz. that there is no such substance at all as germ-plasm, the protoplasm itself being quite able to do all the work that is required.
Microscopic investigations of the germ-cells, which give rise to embryos, exhibit definite structures called "chromosomes," which enter into the first cell of the offspring, and as there must be something to carry on hereditary tendencies, these are most probably the agents; but their chemical constitution is unknown.
Dr. Weismann contends that if the deep-seated germ-plasm could be influenced by external agencies, then their effects might be retained and hereditary; but as a rule the soma intervenes and would prevent any such influences from affecting the germ-plasm, the existence of which, however, has never been optically proved.
The question, then, which Weismann and his followers give to be answered is: Can any change of form and structure induced by external causes in the soma of a plant be transmitted to the next generation irrespective of conditions?
The only reply is to see what nature does, and to test it by experiments. These supply two lines of proof, inductive evidence and experimental verification.
It has long been noticed that the floras of countries with pronounced climatal conditions are characterised by having a more or less general or common facies. Hence we are familiar with the terms arctic and alpine plants, remarkable, for example, by their dwarf character and the brilliant colours of many of the flowers. Then there are moist tropical floras, hot desert plants, and so on.
Now all the plants growing under such several onditions reproduce the features characteristic of the genus, species, &c., by seed; so that they are obviously hereditary, as long as they live under the same conditions.
Do they change if taken away from their natural surroundings and are, say, cultivated? As a rule they do not at first show, to any great extent, any alteration in strongly marked features. Thus seedling Opuntias, Cactuses, &c., retain their characters if raised artificially.
On the other hand, many plants begin to change at once. Thus naturally hairy plants may become hairless, fleshy maritime plants may acquire ordinary thin leaves away from the coast.
Similarly, aquatic plants if crowded will often thrust up branches into the air, the anatomy of which at once changes at the water-level in adaptation to an aerial existence.
The next question is, what are the causes which give rise to any peculiar facies? Why are so many plants fleshy in dry countries, like the Cactaceae of Mexico and the Euphorbias, Stapelias, and Mesembryanthemums of South Africa?
Or, on the other hand, why have so many dicotyledonous plants finely dissected leaf-blades when growing under water? Is it drought in the first case, and water in the second case, that are the causes ?
Inductive evidence means that there exists a vast accumulation of independent cases, among plants of no affinity, and situated in diverse parts of the world, whether on land or in water; so that there arises a "moral conviction" that there are to be seen distinct causes and effects. This line of argument leads one to the conclusion that it is infinitely probable that drought and water are the respective causes. It is the argument accepted by physicists who believe they know the elements in the sun, but they cannot experiment upon the sun itself to corroborate their inductive evidence, supplied by the spectroscope. Astronomers believe that the earth rotates on its axis to produce day and night; but they cannot prove that the sun does not go round the earth in twenty-four hours. It is solely that it is infinitely more probable that the earth should rotate than that the sun should go round it. The result is not a mathematical demonstration, but a moral conviction.
If now the peculiarities of plant-structures were acquired in response to the action of the environment, then it is obvious that they are hereditary.
Although inductive evidence is really ample to substantiate the belief, it has been thoroughly corroborated by experimental verification. Indeed, we need not go beyond the kitchen garden, for if we compare our Carrots, Radishes, Parsnips, all the Cabbage tribe, &c., with the wild plants from which they have been derived, we see at once that their peculiarities, confined as they are to the vegetative system or soma, are all acquired characters, and arose by the stimulating action of a rich and artificially prepared garden soil.
There is no question of germ-plasm of the reproductive organs being concerned in it; because the enlarged roots, the altered forms of "greens," Brussels Sprouts, Kales, even Cauliflowers, are all formed long before the reproductive organs put in any appearance at all. Yet we know that these garden forms of acquired somatic characters are hereditary, for they all come true by seed.
Turning back to the question, "Will plants transmit any acquired character to the first generation?" the answer is, judging by horticultural experience, any new character which the cultivator wishes to preserve must be perpetuated by selection and by using the same external conditions of soil, &c., for five or six years before he can depend upon its fixity. As a rule one year is not enough. Nevertheless, it sometimes happens that variations arise which are permanent from the first year of their appearance, as, has occurred with certain strains of Chinese Primrose, species of OEnothera, &c. It is not, therefore, a fair question to ask for proof that any character acquired in one year shall be transmitted to the first generation; and that too, wherever the offspring may happen to grow. Nature demands accumulation under the same conditions, till fixity can be trusted and heredity secured.
|* Production et Fixation des Variétés dans les Végétaux, par E. A. Carrière; p. 9, 185.|
"Everything tends to become hereditary,"* says a writer on garden varieties, but everything must, as a rule, be encouraged for a few years, when Nature responds to the care of the cultivator and fixes the variation.
Though cultivation supplies us with ample evidence to prove the fact of the inheritance of acquired somatic characters, experiments are not wanting to show what are the causes which have brought about the natural peculiarities of wild plants growing in distinct environments. Thus M. Lesage has proved by experiments that the fleshiness of so many maritime plants, as the Samphire, is directly due to salt.
M. Bonnier, by cultivating lowland plants at Paris, &c,, upon high altitudes in the Alps, proved that they all more or less acquired the dwarf form and peculiar anatomical structure of the leaves, &c., characteristic of alpine plants, as well as the intensified coloration of their flowers. Conversely, normally high alpine plants cultivated at low levels did not change so rapidly, but required some years to do so.
|* Botanical Gazette, xxxiv., Aug. 1902, p. 93.|
M. Eberhardt carried out experiments to contrast the effect of an excessively dry air with those of a very moist one. The results were that the external forms and internal anatomy agreed precisely with those of dry alpine desert plants, and other localities noted for drought, on the one hand, and with the anatomical details of plants growing normally in very moist situations on the other, the structure of the latter being in the same direction as the anatomy of aquatic plants. Lastly, to test the question whether the finely dissected type of leaf in submerged dicotyledoñous plants was due to the water, Mr. McCallum experimented with Proserpinaca palustris, a plant of the United States, which bears fully developed leaves in air, but dissected ones under water.*
Assuming, as a "working hypothesis," that the submerged leaves owed their forms to a surcharged protoplasm, which, by being saturated with water, was too weak, so to say, to produce full-sized leaves, he charged the water with certain nutritive salts. This set up osmotic action in the plant, and withdrew the excess of water from the protoplasm; whereupon the plant forthwith formed full-sized leaves under water.
We have thus now an abundance of proofs, both by induction and experiment, that the form and structure of the organs of plants are due to the immediate response of the living protoplasm to the influences of the environment, and that it, or rather the nucleus, builds up just those cells and tissues which are in adaptation to the conditions of life. Then, after a few years, they become hereditary and so fix the varietal or specific characters by which botanists recognise and distinguish plants in nature.
Whenever these specific characters arise in the vegetative organs, they have been acquired by them, i.e. the soma, long before any reproductive organs were in existence, and therefore before germ-plasm could be present—at least, presumably in sufficient quantity to receive impressions. from without; unless, as already stated, it be as universally distributed. through all the vegetative organs as protoplasm itself. But this supposition at once does away with the necessity for it, for protoplasm and the nucleus are quite able to do all the work required.
If it be further asked, "How can any impression be conveyed from the circumference, where it is received from external impulses and irritations,, to the embryo-sac ?" the reply seems to be that it may be conveyed thereto, by protoplasmic continuity. There is at least a machinery which it is conceivable may he capable of conveying vibrations to the germ-cells when these latter come into existence. But as to the nature of this inherent potency we know nothing, and at present can conceive nothing; though of the fact of characters acquired through the soma, i.e. by the vegetative organs, being subsequently hereditary, there is universal evidence. It may be added that it is perfectly inconceivable how Evolution could proceed a step if it were not so; and that Darwin was undoubtedly right when he regarded inheritance as a sine qua non in the origin of species. Dr. Weismann was right in saying, "My theory rests upon theoretical considerations," but wrong when he added, "and the want of any actual proof of the transmission of acquired characters."
It is sometimes urged that there is no proof that mutilations and injuries, whether to the animal body or to trees &c., are hereditary. Fortunately for man, they are not; but even if they were, they are all beside the question of Evolution. What one is looking for as being hereditary are useful adaptations acquired by the soma, which fit the plant or animal to its new environment better than the parental structures; or else they are organs which become degenerate, as boughs turn to spines in drought, supportive tissues tend to disappear in submerged plants, and floral organs become "rudimentary."
Mutilations, injuries, or diseases are not favourable or useful variations of structure, nor are they natural degenerations. Hence all references to such have nothing whatever to do with acquired variations which a systematist regards as characteristic of varieties and species.