In a way completely different from the second-in-time of a photograph, when you paint from live specimens, you form a kind of relationship with the plant as it slowly changes during the painting process. You are then able to more completely capture the plant's essence and complexity. Sometimes a painting can take years to complete if the artist has to wait for the plant to rebloom.
Even the technique of painting botanicals cannot be rushed. It is a slow and patient process of glazing many layers of seemingly unrelated colors one over the other, waiting for each layer to dry before adding the next one, until the perfect glowing color is achieved. The natural transparency of watercolor allows light to pass through the paint layers, bounce off the white of the paper and reflect back through the many layers of color so that they blend together, becoming a new color altogether. This creates a subtlety and depth of color not possible in other mediums or techniques. I find that the liveliness of watercolor makes it particularly appropriate for capturing the life force of botanicals.