FERTILISATION. 259 



meeting the entering tubes ; but the tiny cavity is 

 turned round from the centre, to face the wall of 

 the ovary (F), which is slightly hairy within. 

 Clinging to this hairy surface, the tubes feel their 

 way along, to find, and at once enter, the point they 

 seek. The helper cells (k, Fig. 50) now disappear, 

 while the embryonal vesicle becomes granular, and 

 two nuclei can be detected in it. One of these is 

 the nucleus of the oosphere, or embryonal vesicle ; 

 the substance of the other has, doubtless (according 

 to Sachs), been derived, through the helper cells, 

 from the pollen tube. These two nuclei, male and 

 female in their origin, meet and coalesce, constituting 

 the nucleus of the fertilised embryo, or new indi- 

 vidual, which now surrounds itself with a cellulose wall, 

 and so starts an existence, which yet depends upon 

 nurture derived from the female parts of the parent 

 flower. When it has acquired some development, 

 and a supply of food sufficient to enable it to initiate 

 a separate existence, it will be cast off as a 

 mature seed. 



We have, up to this point, spoken of flowers as 

 though they invariably carried both stamen and pistil, 

 and usually this is the case ; but exceptions are not 

 infrequent. Every one knows that, in the melon, 

 vegetable marrow, cucumber, and other plants belong- 

 ing to the order Cucurbitacex, some of the flowers 

 are male, while others are female, the latter bearing 

 the fruits. In these cases, it is obvious that the pollen 

 necessary for the fertilisation of the ovule must be 

 carried, by some means, from one form of flower to 

 the other ; and when, by the method of culture, insects 



Y 2 



