ro6 Morphology BOOK I 



The passage of the pollen tube down the tissue of the 

 style was first shown to be the expression of a true process 

 of germination by Van Tieghem in 1871. He pointed out 

 that the pollen grain contains small stores of nutritive 

 materials, and that the tissue of the style is similarly 

 supplied. Mangin showed in 1886 that the germination of 

 the pollen grain is accompanied by very active respiratory 

 changes. In 1893 the present writer was successful in showing 

 that the progress of the growth of the tube is due to digestive 

 changes set up and maintained by enzymes secreted both 

 by the pollen grain and by the tissue of the style. 



The mode of passage of the contents of the pollen tube 

 into the embryo-sac was held by Sachs, as we have seen, 

 to be a process of diffusion. When further details of 

 the behaviour of the synergidae had been obtained, it 

 was evident that the fertilizing substance is not liquid, 

 and that, therefore, bodily entry into the embryo-sac 

 must be effected. Van Tieghem, in his paper already re- 

 ferred to, claimed to have seen a perforation of the 

 pollen tube, sometimes single, sometimes occurring at 

 many points just behind the apex, with small drops of 

 viscid matter exuding. 



The writer observed in the pollen tube of Narcissus 

 a well-defined aperture in a particular position on one 

 side of the tip. Similar appearances were observed by 

 Ewart in 1895, but he concluded they were temporary 

 rather than permanent openings. They were apparently 

 not due to accidental rupture. In 1897 Schaffner studied 

 the pollen tubes of Sagittaria, and observed a thin area 

 at their tips, which became ruptured after the tube had 

 penetrated the embryo sac, so that the two nuclei were 

 discharged from it. V. H. Blackman showed in 1898 that in 

 the Gymnosperms the tip of the tube becomes fused with 

 the egg membrane and does not enter it. 



The first description of the structure and mode of develop- 



