98 GERMINATION OF POLLEN [CH. 



where the outer wall is thick and strong. In both cases 

 the similarity is heightened by the outer walls being 

 cuticularised, whence there would be difficulty in the 

 absorption of water by the protoplasmic contents but 

 for these permeable spots. Not all pollen-grains have 

 so highly differentiated an outer coat, however, e.g. Zostera 

 and other NaiadaceaB as well as some other aquatic 

 plants, and the grains of Orchid pollinia, &c., are devoid 

 of a definite exine, and in some cases the differentiation 

 into two coats is either very slight (e.g. Senecio) or confined 

 to special areas (Onagracece, Cobcea, &c.). 



When the pollen-grain is placed in a drop of dilute 

 sugar solution, such as is found naturally on the stigmatic 

 surface of an ovary, or may be prepared artificially, and 

 the following processes traced step by step with the 

 microscope, a series of changes occur which justify still 

 more our comparison of the grain to a spore ; for the 

 pollen-grain is a spore, and shows its nature now by 

 acting as such. 



The protoplasm in the interior of the ripe grain when 

 first sown is, as we have seen, a granular mass of cyto- 

 plasm with a well-developed nucleus in it, as well as 

 variable quantities of oily or carbohydrate food -materials. 

 This now absorbs water and swells, and its nucleus 

 divides, and a delicate cell-wall may or may not be 

 developed between the two nucleated masses of proto- 

 plasm which result from the division. This wall is so 

 arranged that it cuts the pollen-grain into two unequal 

 cells. The larger of the two is the vegetative cell, and 

 the smaller the generative cell of the pollen-grain. 

 Meanwhile, the pressure of the swelling protoplasm is 

 stretching the exine more and more, and the vegetative 

 cell, feeding on the sugar, oil, and other food-materials 

 as it does so, grows out as a tubular prolongation 



