Naturalis rather a rare occurrence, because the chances of a male motile cell travelling from its own prothallus to that of another species must be very small indeed. However, it does occur occasionally
Artificial hybridization may be attempted when the combination of some desired characters of two species or varieties may promise something good, or, of course, it is used for experimental purposes and research. The simplest method is to mix theof two varieties very thoroughly and sow fairly thickly. When the prothalli are at the correct stage irrigate them forcefully with a fine jet of water from a syringe of suitable size. A chemist’s washing bottle is a useful weapon. This should be done on successive days for a few days, in the hope that the male sperms from one variety may be washed into close contact with the prothalli of another.
Another method is to immerse the pan of prothalli in water at 70° F. for half an hour, so that the prothalli are just awash. The warm water induces rapid liberation of the sperms from their antheridia, so that they swim vigorously among the prothalli until they reach he archegonia and effect fertilization. This should be repeated on a few occasions to give every chance for successful fertilization.
Naturally this is something of a hit-and-miss method, and the ratio of hybrids to pure species is likely to be small. Varieties having the same chromosome number, if this is known, should be used — it is little use trying to hybridize a diploid with a triploid.
A further complication with this method is that the different prothalli may mature at different times, and here the keeping of records of times of germination will help. It would be possible to sow the later-maturing variety first, then oversowing with the second earlier maturing variety later.
The most scientific method is based on the fact that the male organs, antheridia, and the female organs, archegonia, are to be found in different areas of the prothallus, and it is possible to remove the basal areas of one set of prothalli, where the antheridia are situated, leaving the wider heart-shaped apical area where the archegonia are to be found in situ. The archegonia usually are sited just behind the notch in the ‘heart’. The basal areas of the other species of prothallus are then taken and carefully placed in contact with the forward areas of the first species and kept close until established.
Irrigation with aired water will assist in liberating the male sperms in quantity, and there is a good chance of successful hybridization.
Another method is to sow two lots of each variety, removing the antheridia-bearing parts from one set entirely. The other set is used to obtain the male sperms only by immersing the prothalli of each variety in a little aired water in a clean saucer. The water will then become full of motile sperms which then can be applied to the appropriate female parts of prothalli which were left undisturbed in their respective containers.
A new safety razor blade is a useful instrument for this plant surgery. Success depends on meticulous care being taken at all stages, and in using material at the right time.
Sample prothalli should be taken and their precise condition noted by examination under a low-power microscope, to see whether the antheridia and archegonia are mature enough for the successful mating of the two species. A hand lens is not strong enough for this examination. Even with this method it is impossible to be sure that all the antheridia have been separated from the archegonia, as the former sometimes grow among the archegonia, but there is a much better chance of hybridization taking place. Records should be kept of all operations involved for future reference and analysis of results. These records will be invaluable if much experiment is undertaken.