On the subject of drawing with science in mind, I thought I would also mention the Margaret Flockton Award held every year at the Botanic Gardens in Sydney. Lots of useful comments there on what makes a good botanical illustration and the prize money is a considerable amount in our genre of art.
Look out also for the new RHS Botanical art show Saturday 20th March and Sunday 21 March. With 40 artists on display and the orchid show on at the same time, it looks to be well worth the trip. Six RHS Gold medalists are the subject of an exhibition, Gold Standard, at Fulham Palace until 12 April. The gallery is open Saturday through to Tuesday.
I've added some more links to the UK botanical artists page and a long American list - it seems more and more artists are making their presence felt on the Web.
Martin