A conventional t-shirt will quite happily soak up substantial quantities of perspiration. The problem is that by soaking it up it holds onto it. And pretty quickly you're riding around in a sopping wet garment that's chafing all over the place. Worse, stop riding and you'll get cold very quickly. Which is where base layers come in...
Embers Merino base layers New Zealand's finest wool made into base layers by a Derbyshire company - 16/1/07
Helly Hansen Freeze base layer A mixture of high-tech fabrics and Merino wool makes for a garment intended for stop-start activities. Sounds like our kind of thing... - 21/11/06