The crossing of varieties of wheat, etc. - The essentials of fertilisation - Rimpau's experiments - Hybrids and selected varieties.
In the more hopeful view of the case which the new agriculture will have to take, it will recognise the physiological truth that since the living plant is the important and variable machine which constructs the produce looked for, and since that machine will work best in proportion as its needs are properly satisfied; therefore in cases where the needs of a given type of the machine cannot be efficiently provided for, it will be well to select some other type which will take what supplies and conditions can be offered. Of course, this is already recognised to a certain extent, as is implied in the practices of "rotation of crops," selection of "pedigree wheats" and mixtures of "pasture grasses," and in decisions as to the quality of land according to the kinds of weeds found on it, and so forth; but I am convinced that the agriculturist of the future - and the same applies to the horticulturist, planter and forester - will have to concern himself more systematically with the working and the variability of the plant, and particularly with what Darwin termed Variation under Domestication, than has always been the custom in the past. The subject of the plasticity of cultivated plants, and especially of hybrids, is in one sense an old one; but much work is being done which proves, as such work is apt to do, that very much more may be done by well-planned experiments on the selection of new varieties raised by hybridising and cultivation.
In illustration of this point, a short summary of some of the results of crossing different species of wheat, barley, oats, peas, beet, etc., may serve to show what has been gained and what may be hoped for in these directions. It should be stated that much has been done and is being done in this country as well as abroad, as witness English varieties of corn, peas, and potatoes, and the recent experiments on crossing various kinds of maize in America.
The hybridiser grows his cereals, etc., in pots until ready for crossing, and then takes them into the laboratory, removes the weaker spikelets, and takes out the young stamens from the flowers left on the plant. The female plant is then ready, and the flowers covered with paper caps. The pollen, obtained by a clean wet brush from the plant chosen as the father, is then carefully placed in position on the stigmas, and the caps replaced. The pollination is repeated occasionally, and care taken that no uncrossed flowers develop later. In this way a few seeds or grains are got to start with.
This would be the place to introduce an account of the enormous advances made by the botanists of the last decade or two in the study of the microscopic phenomena of fertilisation. Without going into details - which would more than occupy all the space at command - I may recall the discoveries of Strasburger and his pupils, and of Guignard, which have supplemented the earlier discoveries of De Bary, Cohn, and Hofmeister, by establishing the facts that the essential point in fertilisation is the fusion of two nuclei, and the bringing together in the fused mass of two extremely minute threadlike coiled bodies, the so-called chromatosomes or filaments, one of which is derived from the male and the other from the female parent. The particulars as to the marvellous adaptations to secure the union of these two infinitesimally minute threads, their behaviour immediately before and after union, and many other points must be passed over, as I have only space to emphasise the one crowning discovery that these tiny filaments of nuclear substance are the material carriers of all the hereditary properties of the parents to the young plant which their union initiates.
It must not be supposed that the above statements are based on any meagre foundation of facts. The attraction of the fusing nucleated masses had been demonstrated over and over again by Tulasne, De Bary, Strasburger and others; but Pfeffer brought the matter to a crisis by discovering the attractive (chemotactic) substance emitted in given cases, and by collecting the fertilising bodies by its means into artificial tubes.
The fusion of the nucleated bodies in the sexual act was observed by Strasburger in the living plant a few years ago, and numerous later observers have confirmed it. Meanwhile all the stages of approach and contact of the essential filaments of the nuclear substance have been traced, as also all the stages of the transference of half of each filament, male and female, into each of the first two cells of the very young embryo-plant.
Moreover, the essentials are found to be the same in the animal kingdom also, and the bearing of all these discoveries on the phenomena of reproduction, variation, and heredity in living organisms has been and is of the highest importance, for they support, control, explain and correct so many of the splendid results of Knight, Kolreuter, Sprengel, Hildebrand and Hermann Miiller, and in every direction throw side-lights into the crevices of that magnificent structure, the theory of Natural Selection, erected for all time by our countryman, Charles Darwin.
To return now to experiments on crossing. It is found that the first products of the crossing appear exactly alike; they may have characters intermediate between those of the father and mother, or they may resemble one more than the other, but all the seeds of the same cross do it in the same way.
On then sowing the seeds of the plants produced from this first cross, variations begin to appear. Most of the progeny revert to one or other of the parent forms, others show all conceivable combinations of their characters, and a few may give rise to entirely new characters. In succeeding generations the reversions are preponderant, and, supposing no care is taken to prevent it, the whole of the offspring gradually go back to the ancestral type.