This month, Lou and I visited Longwood Gardens in Kennett Square, Pa., following a short trip to Philadelphia.           Most of the outside gardens were still dormant, so we stuck to the Conservatory and took in the Orchid Extravaganza that was on display. The Conservatory was breathtaking, and I loved the thoughtful integration of plants within the light-filled architecture. Leaving the soggy, gray day behind and entering the balmy temperature and oxygen-rich air of the Conservatory, it was immediately summer.