Collins Guide to Tropical Plants, Wilhelm Lotschert and Gerhard Beese, 256 pp., including 274 color photographs (grouped on 96 pp. in the center of the book), 5 x 7¾ , index, Steptlen Greene Press, Lexington, Mass. $24.95. The subtitle says A descriptive guide to 323 ornamental and economic plants, so obviously it can cover only the most prominent and common plants. (There are about 3,500 varieties of palms.) A one-page map of the world shows the climate and vegetation zones of the tropics, which, with a two-page description, gives some guidance as to the plants to be expected in any region. For each species covered, the book gives a botanical description, with flowering time, origin, distribution and requirements, and for economic plants...