184 



STRUCTURAL BOTANY 



very conspicuous to the visitors by their great size and 

 bright colours, and in some species the sweet smell is 

 an additional attraction. Generally the style projects 

 so far that an insect comes into 

 contact with the stigma before 

 touching the stamens. Hence it 

 will usually bring pollen from the 

 flower which it has last visited, 

 and will thus effect cross-fertilisa- 

 tion. But sometimes it happens 

 that the anthers and stigmas oi 

 the same flower brush against each 

 other, and then self-fertilisation 

 may result. As the visits of the 

 insects are rather uncertain and 

 very dependent on weather, this 

 power of fertilisation by its own 

 pollen is very useful to the Lily 

 as an extra chance of setting its 

 seeds. 



When the pollen-grains, by one 

 means or the other, are brought 

 into contact with the stigma, they 

 stick to its viscid surface and soon 

 begin to send out their tubes. 

 The grain takes up water from the 

 stigma, and begins to swell. The 

 fold of pure cellulose (see p. 178) 

 bulges out, and, following the 

 growth of the protoplasm within, forms the membrane 

 of the pollen-tube (see Fig. 80). 



/an. 



FIG. 84. Embryo-sac 

 of a Lily, s, s, syner- 

 gidae ; o, ovum ; v, v, 

 vacuoles ; p l and p. 2 , 

 the polar nuclei ; 

 an, antipodal cells. 

 Magnified about 250 

 diameters. (After 

 Guignard. ) 



Note that this figure is 

 reversed as compared 

 with Fig. 83. 



