CHARLES Rennie Mackintosh is a classic example of what happens when an artist's apparent popularity obscures his genuine achievement. Working principally in Glasgow at the turn of the 20th century, he created buildings, furniture and jewellery which synthesised high Victorian style with the Arts and Crafts of William Morris, Whistlerian Aestheticism and trends in European art, including Art Nouveau.

He was also the author, as was seen in last year's major National Galleries show, of superb large-scale landscape watercolours. Mention his name the world over today and you'll receive an immediate nod of recognition. But I wonder how many of those people who claim an affinity with his work would recognise real Mackintosh if they saw it? Now, however, an ambitious and wide-ranging festival filling Glasgow's galleries this autumn looks set to repair part of the damage.

Mackintosh was the driving force behind not just what we know as the Glasgow Style, but also the popular image of what modern Scottish style is all about - a streamlined, marketable reworking of the old Celtic myths and motifs. A style which lives on, bowdlerised in gift shops, imitated to the point of derision. Mackintosh has a lot to answer for. But he didn't do it single-handedly. And that is one of the major lessons of this festival. Mackintosh worked closely with a number of collaborators - including his wife Margaret - and two of these associates are the focus of the event's most original and absorbing shows.

Doves and Dreams examines the work of husband and wife team Frances Macdonald and James Herbert McNair. She was Mackintosh's sister-in-law, he an architect colleague of Mackintosh, and together the two couples have come to be known as 'the four'. The roots of their friendship were formed in 1889, when Mackintosh joined the architectural practice of Honeyman and Keppie, where McNair had been employed for a year. By 1893, they had met the Macdonald sisters at evening classes at Glasgow School of Art. Charles and Margaret married in 1900, the year after Frances and James became husband and wife.

While all four should doubtless be grouped together, as this exhibition ably demonstrates, their individual style and capabilities diverged considerably within the school as a whole and to understand how, you would probably do best to visit some of the other shows and view at least one example of full-blown Mackintosh design, such as the House for an Art Lover in Glasgow, before coming here. The School of Art currently has a good display of Mackintosh's architectural drawings on show, and the Hill House in Helensburgh is also showing rarely seen pieces from the collection of the National Trust for Scotland.

Certainly, the accepted importance of McNair and the Macdonald sisters to Mackintosh has been that they encouraged him to pursue an art which drew on symbolism and the language of dreams and myths. Without them he might easily have remained the prosaic if inspired architect that he had become by the mid-1890s.

In a sense they enabled him to integrate his discursive watercolours into the naturally expansive character of his buildings. This show demonstrates for the first time, however, that they were more than mere muses to his genius, but significant artists in their own right.

AS MUCH IS EVIDENT from the first room, whose principal focus is Frances Macdonald's menacing watercolour entitled A Pond, dating from 1894, when she must just have encountered Mackintosh and McNair. It is not hard, looking at this and similar works, such as The Creation of Eve of 1893, to see that the root of her art - and that of her sister's - with its yearning, attenuated forms and mystical mythical subject matter, lies principally with Burne-Jones and the late Pre-Raphaelites.

Within 10 years, however, both sisters had helped to create a newly refined style of decorative art, in tune with the prevailing zeitgeist. In 1900, the McNairs went to live and work in Liverpool and the course of their life and legacy in that city is nicely explained here in interpretive panels. We also experience the essence of their style in a perfect recreation of the Lady's Writing Room, which they exhibited in Turin in 1902. Even without this, the show is an unmissable opportunity to see so many works by the couple gathered together with the clear intent of provoking new thoughts on their relationship to Mackintosh, and also of reminding us of their salient place in British art history.

Looking at the graphics, the latter is evident through the curious ability of these works to appear to be creations of the 1960s or '70s rather than of 70 years earlier. The reason, of course, is that during those two decades the Glasgow Style, along with Aestheticism and Art Nouveau, were taken up by style gurus and imported into the popular marketplace. Nevertheless, no matter how often you remind yourself of this fact, for anyone who grew up with Biba and Yes album covers, the images have an absurd and utterly anachronistic familiarity.

This is true not least in terms of their raw sexuality. Frances Macdonald's watercolour The Lovers, from 1893, is about as explicit an image as you will find in Victorian Britain outside the realms of intended pornography, and as you progress through these rooms with this in mind the eye becomes accustomed to discerning decidedly priapic forms.

Again and again, the faces of the Macdonald sisters' maidens pout and gasp in specifically sexual ecstasy. Here are the forerunners of JD Fergusson's unrestrained paeans to love and fecundity; celebrations of the power of procreation which are often demonstrably biological. Look again now at the furniture, jewellery and silver which adorn these rooms alongside the watercolours and graphics, and you instantly find further examples of this impetus. Notice how on a clock face made by the sisters in 1896, the woman on the right features a womb-like circle from which or into which flows an unspecified form or liquid. Often the signature stylised lilies of their work closely resemble a vulva, which is also echoed elsewhere in the form of a mandorla.

With all this in mind look at the central motif of a huge poster for the Glasgow Institute from 1896. It might be taking things a little far to suggest that the twining plantain vines and tendrils can be interpreted as fallopian tubes, but think about it. Dressed up as fairies and allegories, the real significance of all this apparently innocuous imagery was at the time strictly private, but here its true meaning has never been more evident.

Take this to its logical extreme, as reinterpreted in Mackintosh's surviving buildings, and you could rightly claim that many of Glasgow's finest buildings and a style admired worldwide for its simple organic beauty, are actually replete with specific, explicit sexual references. Such an interpretation is reinforced by a unique group of works in the final room, devoted to Frances Macdonald, who died in 1921 - possibly by her own hand - and whose presence haunts the show. Painted in 1911, these seven exquisite watercolours have underlying themes of love, sex, birth and death, which at times become subtly erotic. One in particular, entitled Man Makes The Beads Of Life But Woman Must Thread Them returns to the earlier central theme of reproduction, marrying the physical to the allegorical with a poetry and delicacy of touch which marks out the artist as being somewhat more important than she might previously have been conceived.

While an exhibition centred on Mackintosh himself would have been an obvious, money-spinning crowd pleaser, to stage such a show as this as a centrepiece of the festival is nothing short of a masterstroke. It is an unequalled opportunity to experience not just the talent of one man, but the driving spirit of a movement. That is not to deny Mackintosh's own importance of course. But his real legacy is his abiding contribution to the architectural heritage of Glasgow and Helensburgh, and the biggest triumph of this festival is simply to remind us that there is no better way to understand his remarkable achievement than to simply look around ourselves.

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