Observation on the formation of ovule tapetum in Pinus tabulaeformis Carr

The formation process of ovule tapetum in Pinus tabulaeformis Carr. was observed with the paraffin sectioning method. After the first dormancy breaking of the female cones, there are some young cells with similar morphology in the deep center of nucellus. The initial tapetum which has 1 or 2 layers of cells and is dyed deeper in irregular shape appear in the nucellar tissues around the megaspore mother cell. During the meiosis of the megaspore mother cell, the tapetal cells which are apparently different from the surrounding nucellar cells increase to 5-6 layers, and some of the tapetal cells near the nucellar tissues elongate radially. At the two-nucleate female gametophyte stage, 2 layers of the tapetal cells near the female gametophyte degrade, and the tapetum reduces to only 3-4 layers. With the number of free nuclear increasing, the large central vacuole in female gametophyte becomes larger, and the number of the tapetal layer increases to 5-6. About 3-4 layers of the tapetum near the nucellar tissues apparently elongate, and some of them begin to degrade. One or two layers of the tapetum near the female gametophyte also tend to elongate. Until the female cones enter the second time dormancy, the number of layer of the tapetum no longer increased. Because the tapetum which is adjacent to the female gametophyte is developed from the nucellar cells in P. tabulaeformis, it should be called nucellar tapetum.