92 CONTENTS OF POLLEN-GRAIN [CH. 



ripe pollen-grain contains not only starch and oil in its 

 rich protoplasm, but its nucleus has undergone complete 

 division into two, usually of different shapes, and each 

 of which has very different functions : occasionally a 

 delicate cell-wall is formed between the nuclei. 



Apart from certain differences in shapes and sizes, 

 which however are not greater than occur in the Angio- 

 sperms, the stamens of Gymnosperms show essentially the 

 same development as that above described. In each 

 pollen-sac, the number of which differs in the different 

 groups, a spore-producing mass arises as before from the 

 division of cells below the epidermis, and a layer of cells, 

 which eventually become absorbed, is formed around it. 

 The pollen mother-cells formed by division of the arche- 

 sporium divide each into four pollen-grains, and these float 

 in the fluid of the sac as before. 



At the stage where the pollen-grains approach 

 maturity, and are ready to escape by the splitting of 

 the anthers, the essential differences begin to be apparent ; 

 for in the Gymnosperm pollen-grain the nucleus not 

 only divides, but its divisions are followed by complete, 

 though delicate, cell-walls formed across the hitherto 

 single cell. 



The cases vary in detail, but the following example 

 is typical. The nucleus of the young pollen-grain divides, 

 and a curved cell-wall cuts out a small nucleated cell 

 from the larger one, and this small cell undergoes one 

 or two further divisions. 



The ripe pollen-grain thus constitutes a cell-complex. 

 The large cell is in all these cases known as the vegetative 

 cell, and it is this which gives rise to the pollen-tube, and 

 of the other cells the one next to and in contact with the 

 vegetative cell is known as the generative cell. 



As regards shape, pollen-grains differ greatly in 



