A double from David Clarkson sent Motherwell into the fourth round of the Tennent's Scottish Cup at the expense of St Johnstone.

The talented young striker, just 18, seems certain to emerge from the shadow of James McFadden and Stephen Pearson as Well's latest young hot talent.

He opened the scoring in the 24th minute with a perfectly executed lob over the head of Saints goalkeeper Craig Nelson after Brian McLaughlin was short with a clearance.

Clarkson struck again, nine minutes later, punishing Billy Stark's men for some woeful defending with a well-taken shot at the back post.

The result was confirmed in the final minute when debutant Scott McDonald pounced to score his first goal for the club after being played in by Paul Quinn.

Saints, a team with significant top level experience, were never in contention and were lucky not to have lost by a wider margin.

Stark handed new signing Jamie McQuilken his debut, in place of Paul Lovering.

Highly-rated youngster Mark Baxter was also recalled to the side, replacing former Hibs defender Darren Dods.

Striker Kiegan Parker, who was offered to Aberdeen in midweek, dropped to the bench to make way for Paul Bernard.

Motherwell started with in-form striker Clarkson, who scored his first senior hat-trick against Dundee United last weekend, and Burns in attack.

It was the home side who created the first real chance, after 15 minutes, when McLaughlin burst into the box and sent a shot ricocheting off Keith Lasley's leg.

The ball then broke perfectly for Bernard, who was positioned on the edge of the area, but Well goalkeeper Gordon Marshall beat away his powerful drive.

Motherwell took the lead in the 23rd minute through Clarkson, who once again demonstrated his excellent finishing ability.

Burns first tried to pick out Lasley with a deep cross-field ball, but saw it intercepted by McLaughlin - who could only hit it out as far as the Well striker.

Clarkson looked up, spotted Nelson off his line and sent a delightful chip sailing over the goalkeeper's head into the net.

The Well striker found the net again nine minutes later, and once again Burns and Lasley were heavily involved in the build-up.

Burns's cross from the right wing dropped perfectly for Lasley, who controlled the ball on his chest before slipping it to his left for Clarkson.

The 18-year old once more showed a level of maturity beyond his years by simply killing the ball stone dead, looking up and then blasting a shot high into the net.

Stark was obviously unhappy with his team's inept first-half showing and made a change at the interval - replacing Mixu Paatelainen with Parker.

Burns and Clarkson were proving too hot to handle for the Perth men and they carved out another opportunity only five minutes into the second half.

Clarkson bolted into the box from the right flank and slipped the ball into the path of his strike partner, who was standing 16 yards out.

Burns's shot was heading for the bottom corner, but Nelson managed to throw himself to the ground and block.

Stark then made another change, bringing on new signing Stuart Taylor - who joined from Partick Thistle on Friday - in place of McLaughlin.

Motherwell seemed able to cope better with the swirling wind and came close once again in the 70th minute.

The impressive Clarkson showed another dimension to his game by sending in an excellent cross for Burns at the back post.

Unfortunately for the small forward, he was unable to meet the ball with a diving header.

The Steelmen were in complete control as the game wore to its conclusion, playing out time and allowing Terry Butcher to give the large travelling support what they demanded - the introduction of Phil O'Donnell in the 80th minute, in place of Derek Adams.

They confirmed their superiority in the final minute when McDonald struck from 10 yards out to open his account after moving to Fir Park last week.

Teams

St Johnstone Nelson, Baxter, Maxwell, Robertson, Mcquilken, McLaughlin (Taylor 64), Donnelly (Hay 74), Reilly, Bernard, Paatelainen (Parker 46), MacDonald.

Subs Not Used: Dods, Cuthbert.

Booked: Baxter.

Motherwell Marshall, Corrigan, Craigan, Quinn, Hammell, Lasley (Fagan 90), Leitch, Dair, Adams (O'Donnell 80), Burns, Clarkson (Mcdonald 74).

Subs Not Used: Corr, Kinniburgh.

Booked: Hammell.

Goals: Clarkson 24, 32, Mcdonald 90.

Att: 4,090

Ref: K Toner (Scotland).