The study of plants is built around the idea of a species. Members of the same species sexually breed offspring similar to themselves. They usually have a built-in resistance to cross- breeding with other species, and this maintains the purity and distinctiveness of the species. Occasionally these barriers are broken down and cross-breeding produces hybrid offspring. The hybrids are often intermediate between the parents in appearance; they often have 'hybrid vigour' and spread prodigiously by vegetative reproduction or by seedling swarms.

Interbreeding between plants of a different genus or family is very rare - so a primrose would not cross with a daffodil - but a cross within a genus is more likely, so the primrose Primula vulgaris may well cross with the cowslip Primua veris to produce hybrid offspring Primula veris x vulgaris which is also known by its own name Primula x variabilis.

Strong resistance is developed to crossing with other species which normally grow in the same locality, but less resistance to strangers which intrude into the habitat. Thus introduced garden or agricultural plants may hybridise freely with local wild plants. For similar reasons hybrids occur in nature at places where two distinctly different habitats meet - such as the edge of a wood.

A recent compilation of hybrid information for the British Isles (Stace 1975) has made it possible to comment on the likelihood of encountering hybrids in Caithness plant population. Given below are the hybrids which have been found already and some which are expected either because parents known to cross easily grow together, or because the climatic conditions appear to be favourable. Some obvious hybrids have been omitted because their hybridity is of no interest, for example the hybrid saxifrage London Pride (Saxifraga spathularis x urbium) which escapes from gardens where the hybrid was planted. Also only formally recognised records are given, because there are now a few specimens gathered and awaiting proper checking.

The horsetail hybrid Equisetum arvense x fluviatile is widespread in Britain and occurs in Sutherland, Hebrides and Shetland. Is it being overlooked?

The two male shield ferns Dryopteris filix-mas and Dryopteris pseudomas occur in abundance and they are said to frequently hybridise. However it will be very difficult to recognise and it is easy to understand why no record exists.

In the same genus are the two Buckler ferns Dryopteris carthusiana and Dryopteris dilatata. They hybridise regularly in Britain. The Caithness population of D. carthusiana is very confusing and there is the possibility that it is a long-standing introgressive mixture: further investigation is needed. Meanwhile no formal record exists.

Another closely related pair of ferns are the two species of polypody. Polypodium vulgare grows in rock crevices while Polypodiurn interjectum grows on tree trunks. Where man builds a wall round a planted wood there is a conjunction of habitats and Polypodium vulgare x interjectum may be found. There is a record by E. R. Bullard near Reay 1974.

The watercress population is a mixture of the two species Rorippa nasturtium - aquaticum and Rorippa microphylla with the Rorippa x sterilis. Records for all three exist but the relative proportions in the population are not known.

Red campion is sometimes found with pale pink or white petals. There can be two reasons for this, one being that pale varieties of pure Silene dioica occur in the north, the other being that it has hybridised with the white campion to give Silene alba x dioica, and there is a valid record for that plant

The hybrid gorse Ulex europaeus galii is to be expected where the two occur (Achavanich and Mid-Clyth) and one of the existing specimens of Ulex gallii clearly has a 'touch of the tar-brush'. No formal record exists.

One of the prettiest of the half-breeds is to be found where the water avens (Geum rivale) lives alongside the wood avens (Geum arbanum). It has been found along the Dunbeath Water and Thurso riverside, where woodland and marsh come together, by J K B in1974.

Roses are given to hybridity, which adds to the difficulty of understanding a genus which is complex and not well understood. No useful work whatever has been done on Caithness roses and it remains an unexplored territory.

The two common sundews are the round-leaved (Drosera rotundifolia) and the English sundew (Drosera anglica). Their hybrid has been reported but not confirmed. It is occasionally found in Sutherland and is to be confidently expected in Caithness although it is not an easy plant to recognise.

The widespread and prodigious occurrence of willowherb hybrids is well-known to botanists. Yet none are recorded for Caithness. We might reasonably expect Epilobium hirsutum x parviflorum, E. hirsutum x montanum, E. montanum x parviflorum, E. montanum x obscurum and E. obscurum x palustre.

One of the most spectacular introductions to the county flora from gardens is the ten-feet-tall giant hogweed Haracleum mantegazzianum. It crosses freely with the common hogweed Heracleum sphondylium to produce a range of peculiar offspring ranging from two to ten feet tall which are always found in the locality of the introduction. (CSSF record 1972).

The cross between the broad-leaved dock and the curled dock is common throughout the British Isles, but although these two grow commonly in the county Rumex obtusifolius x crispus remains unrecorded.

The birch trees are divisible into two species - the downy birch (Betula pubescens) and the silver birch (Betula pendula) but intermediates are common wherever the two meet. The natural tree of this area seems to be the downy birch but plantings of the other species are common. However no study of the birches has been made and no hybrid record exists.

Willow trees are so frequently planted into habitats and form hybrids with the local residents so easily that it is even difficult to know the origin of some of the specimens that are found. It is only because they propagate more easily by vegetative division than by seed that there are any pure species around to make the situation comprehensible. Two of the possible crosses have been formally noted - Salix caprea x cinerea ssp oleifolia (JKB 1974) and S. herbacea x repens (CSSF 1972). S. cinerea x aurita and S. aurita x repens are to be expected also.

An old record exists somewhere for the cross between the bilberry and the cowberry (Vaccinium myrtillus x vitis-idaea) ; but it has not been seen recently.

Where primrose and cowslip meet (it can only be at Reay) the false oxlip will not be far away. There is a record for it recently.

Two forms of comfrey are common in the county, mainly having been planted to ease sprained ankles and feed livestock. The creamy flowered one is clearly Symphytum tuberosum, while the blue flowered one is a hybrid of uncertain origin, most likely it is Russian comfrey (S. asperum x officinale) but no material has been submitted to proper identification.

The large flowered water forget-me-not (Myosotis scorpiodes) produces a vigorous hybrid when crossed with the smaller flowered one (Myosotis caespitosa). Although there are few British records it is the opinion of national expert P. W Benoit that it is widespread. The two species grow together frequently in the county.

The monkey flower (Mimulus) is an interesting case. It has been introduced from Western N. America since 1830 and spreads vegetatively along the banks of rivers. The whole of the Caithness population seems to be a sterile hybrid Mimulus guttatus x luteus with varying amounts of the M. luteus influence causing a range of red blotch patterns. There is no sign of the parent species, for all the plants that I have examined have not developed seeds. It seems likely that all the records for Mimulus guttatus are incorrect. Can you find a monkey flower with seeds on it?

The eyebrights of the far north are notorious for their complex hybridity, and of the ten recorded species it must be presumed that they all cross with each other to form complex polyglot swarms. In fact only one hybrid has been formally recorded, mainly because most of the material submitted for identification cannot be precisely named.

Perhaps you can imagine the difficulties in sorting out the mints. The two natives water mint and common mint form a hybrid (Mentha aquatica x arvensis) which has not been reported. Then the garden escapes, with true hybrid vigour, have spread into the wild places. They are all hybrids crossed to give the best culinary results, and the ones we know about are:

The most common woundwort in the county is the vigorous Stachys palustris x sylvatica which characteristically grows in large swarming clumps, while the parents are less common and grow single or in small groups.

The pretty little bugles are known to hybridise to give Ajuga pyradidalis x reptans which occurs at Bettyhill in Sutherland. The parents occasionally meet on the east coast of Caithness and could be worth watching.

The ragwort is a serious menace in mature pastures because it is poisonous to animals yet difficult to eradicate. This is not surprising for it is the vigorous hybrid Senecio aquaticus x jacobaea = Senecio x ostenfeldii. Most of the existing records of Senecio jacobaea refer in fact to this plant so that the whole group are now in need of revision. True S. jacobaea is confined to sand-dune areas on the coast. This situation has only recently been recognised (E.R.B. 1973).

The pondweeds can spread as hybrids because bits of broken stem are carried off in the water to take root elsewhere. Potamogeton graminus x perfoliatus = P. x nitens has long been known in Caithness while P. filiformis x pectinatus = P. x suecicus may well occur where the parents grow together.

The two soft rushes Juncus conglomeratus and Juncus effusus form a hybrid - especially in cool upland places apparently - and it seems reasonable to expect that it will occur there. The other two waterside rushes juncus acutiflorus and Juncus articulatus also hybridise readily. No formal records have been made for either of these hybrids.

Orchids hybridise freely and cause much confusion in the naming of the species. The most likely one is Dactirlorhiza maculata x purpurella which is known in a few localities (J. M. Gunn 1974).

One of the most famous of our hybrids is the cross between the Wick sedge Carex recta and the water sedge Carex aquatilis growing on the margins of the Wick river. Its hybrid name is Carex x grantii named after J. Grant of Wick who first noticed it in the late years of the last century. The only known site in the world is Wick river.

Three other sedge hybrids are known: Carex aquatilis x nigra, Carex hostiana x lepidocarpa ssp scotica, Carex binervis x rostrata. In addition Carex demissa x hostiana should occur, Carex nigra x recta is likely and Carex demissa x lepidocarpa is undoubtedly present among the range of variation we see in these two difficult species.

Among the grasses Agropyron junceiforme x repens occurs on seashores (CSSF 1972); Agrostis stolonifera x tenuis is almost certain to occur but has not been recorded. Then there is the case of the orange foxtail; a record for the true orange foxtail was made by R. Brown (a friend of Robert Dick) in 1861 - rather surprising since it restricted to the far south of England.

Following the description of Brown's locality 113 years later JKB found indeed a foxtail which was orange but was in fact the hybrid Alopecurus geniculatus x pratensis. It seems to have lasting qualities!