CHANGES OF ACTIVITY OF PROTECTIVE ENZYMES DURING FLOWER SENESCENCE IN TULIP Studies on Regenerating Bulblets from Tissue Culture of Tulip Studies on Regenerating Bulblets from Tissue Culture of Tulip Distribution of 14C-photosynthate and Changes of Endogenous Phytohormonein the Bulb Development of Tulip ( Tulipa gesneriana L. ) Distribution of 14C-photosynthate and Changes of Endogenous Phytohormonein the Bulb Development of Tulip ( Tulipa gesneriana L. )