The Cambridge University Herbarium
Herbaria are places where reference collections of dried, pressed plant material are kept. They are an extremely valuable resource in studying plants as anatomical details are preserved indefinitely by the drying process allowing plants from around the world to be compared and examined in one place.
The largest herbaria, such as those held at the Royal Botanic Garden Kew and the Natural History Museum, comprise several million specimens and have the study of wild plants as their primary purpose. However, there are a number of herbaria which actively accession material of plants that have arisen in cultivation. Links to some of these are provided below. All of those listed will accept nomenclatural standard specimens.
Details of herbaria from across the world can be found on the Index Herbariorum website.