Preliminary Studies on the Elements of Browning and the Changes in Cellular Texture of Leaf Explant Browning in Phalaenopsis Changes of Total Phenol Content and the Activities of PPO, POD and PAL during the Browning in Phalaenopsis Explant in Vitro Changes of Total Phenol Content and the Activities of PPO, POD and PAL during the Browning in Phalaenopsis Explant in Vitro