THE FERTILISATION OF FLOWERS. 



317 



one pollen-grain may be very numerous, although as a rule only one 

 tube grows from each grain. 



Now, the essence of fertilisation (i.e. the production of a " seed " 

 fitted to produce a new plant) appears to consist in the contact of the 

 pollen-tube with the ovule, so that the viscid matter called/cw///^ con- 

 tained within the pollen-grain, may be applied to the structures of the 

 ovule. The most important part of the ovule itself is a small cellular 

 body called the nucleus, enveloped in a couple of coverings. The 

 hollow interior of the nucleus is named the embryo-sac, and an opening 



FIG. 224. POLLEN-TUBES OF DATURA PENETRATING THE STYLE (MAGNIFIED). 



called the micropyle also exists in the coats of the ovule. Through 

 this opening the pollen-tube passes, gaining admittance thereby to 

 the nucleus, and thence to its hollow body or embryo-sac, wherein 

 the fovilla, or contents of the pollen-grain, are discharged. 



Such is the work of fertilisation, and such are the processes in virtue 

 of which the ovule becomes the seed. As the result of these processes, 



