Other leaders leave to spend more time with their family, after a visit from the men in grey suits, or doing the honourable thing when offered the fabled pearl-handled revolver.

On the morning that Labour announces its fifth leader in 10 years of the Scottish Parliament, the cheerful chieftain of the Scottish Greens is announcing that he is leaving under no pressure at all - except, that is, the desire to get at least some of his life back.

Robin Harper ends his resignation statement with a boyish grin and declaration that he remains an optimist. The politics and economics of the future must be Green politics if we are to survive and thrive, living imaginatively and productively within our ecological means," he says.

It hasn't always been easy to stay optimistic when the planet looks this threatened, and Greenery has been a hard political furrow to plough.

He joined the party in 1985, on the day French government agents sank Greenpeace's Rainbow Warrior campaign ship in Auckland harbour.

Within seven years, he was in one of the Green's various leadership positions - either as convener, co- convener or "principal spokesperson".

That is the sort of gender-balanced, non-hierarchical idealism he is now telling his party it could do without if it wants to thrive.

If it rejects his case for a single leader, it could be only Green Party in Europe without one. "That would be a bit bizarre," he observes, then thinking out loud that the public might contribute to the party's debate, as they are target voters so their opinions should matter most.

Streamlined leadership is an unexpected message from an MSP who eschews modern managerial politics and spin.

The former history and science teacher's style is more that of a gentle, amiable, thoughtful eccentric who has wandered absent- mindedly into Holyrood's debating chamber from a guitar session in a folk bar - a sort of political Womble, but one with sincere seriousness of purpose.

Mr Harper stood 10 times for councils and parliaments, losing deposits before striking lucky as the sole Green MSP in the 1999 Scottish Parliament elections.

That left him "working with Tommy Sheridan", I vaguely recall.

"No, I was not working with Tommy Sheridan," he splutters indignantly. They shared speaking time, but that was it.

Contrasting with the Scottish Socialists' protest and schism, his job was to show Greens could be responsible parliamentary citizens.

This was the beginning of the rollercoaster. In 2003, seven Greens were elected from around Scotland, and he hints that these colleagues should now form the focus of the new leadership.

But the party was heading for the rocks in 2007, when only Mr Harper and Patrick Harvie in Glasgow remained at Holyrood.

"I was gutted, absolutely gutted," he recalls. "I saw it coming months before, for two reasons.

"Labour were stumbling and the SNP were fighting a cracker, and we predicted the form of the ballot papers was going to cause maximum confusion.

"It was designed to do that because it struck at our highly successful Vote Green 2" campaign from 2003".

He talked before last year's election of bounding up the steps of Bute House to put Greens into coalition government.

But while negotiations with the SNP landed Greens some special access and a committee chair, the refusal of LibDems to join talks put paid to Mr Harper's dreams of using a ministerial bus pass.

Harper hopes his successor can become Deputy First Minister, as the coalition plan remains in place for next time.

The "tremendous inertia" and compromises of bigger parties will ensure Greens always have a place in pushing environmentalism, even when courting unpopularity by urging high energy costs to cut fossil fuel use.

He cites numerous incremental achievements and legal amendments since 1999, but the impact of Greens has been felt most by keeping ministers on their environmental toes.

Robin Harper hopes to return to orchestral concerts, theatre, friends in a debating society, all of them neglected since he became an MSP, and to have more time with Jenny, whom he married 14 years ago.

"I look back and I have nothing to punctuate my work in terms of life experiences," he says.

"I've done my 10 years, I've made a huge commitment, more than most MSPs in terms of my time, and aged 71 at the next election, I don't have endless time. I'll have a wee bit of fun before I'm called to somewhere else."