THE SPRUCE FIR 267 



after dividing up a great many times, gives rise to a 

 large mass of pollen mother - cells. Here, as in 

 Angiosperms, the pollen - producing tissue grows at 

 the expense of the surrounding layers ; the innermost 

 of these layers, called the tapetum, is rich in proteids, 

 and is the first to be digested. The outer wall of the 

 pollen-sac is originally about three cells in thickness, 

 but before maturity all except the epidermis have 

 disappeared (see Fig. 110, B, C). 



The division of the pollen mother-cells takes place 

 in the early spring. Each mother-cell divides into 

 four daughter-cells, which become the pollen-grains. 

 An enormous number of pollen-grains are produced 

 altogether. If we shake a Fir-tree in May, when 

 the male flowers are lipe, the pollen comes out in 

 great clouds of yellow dust, which float for a long 

 time in the air, and are carried to great distances if 

 any wind is blowing. The Fir is a wind-fertilised tree 

 that is, the transport of the pollen-grains from the 

 stamens to the ovules is entirely dependent on 

 currents of air. Hence a very small proportion of 

 the pollen-grains ever reach the ovules, and in order 

 that there may be a fair chance of pollination taking 

 place at all, a very large number must be produced, 

 the vast majority of which are wasted. We will now 

 see why the pollen-grains are so light and float so 

 long in the air. If we examine the ripe pollen-grains 

 under the microscope, we find that each grain has a 

 curious winged form, owing to the development of 

 two large air-cavities in the cell- wall (see Fig. 110, 

 D. w). The wall of the pollen -grain is covered on 



