THE VASCULAR CRYPTOGAMS 69 



make way for the outgrowth. The projecting part of the 

 central cell is presently cut off by a wall, and forms 

 the canal of the neck. This canal-cell may itself undergo 

 one or two further divisions, but they are usually in- 

 complete, no cell-walls being formed. A second canal- 

 cell is now cut off below the first ; the remaining part 

 of the central cell rounds off its protoplasm, and now 

 constitutes the ovum. The archegonium has by this time 

 reached its complete development. The neck is not 

 straight, but is sharply curved backwards, i.e. towards 

 the basal end of the prothallus (see Figs. 35 and 36). 



We see that the archegonia are really quite similar to 

 those of Selaginella, and also have much in common with 

 the archegonia of Conifers. 



0. FERTILISATION 



In Ferns, as in Cryptogams generally, fertilisation can 

 only take place under water. In nature this happens 

 after rain or heavy dew, when the under-sides of the 

 prothalli are thoroughly wetted. When we are cultivat- 

 ing prothalli it is necessary to sprinkle them with water 

 from above, when the sexual organs are ripe, if we wish 

 to obtain embryos. We have already seen how the 

 antheridia open under water, and how the active 

 spermatozoids are set free. In like manner, the 

 archegonia, when moistened, open to receive them. 

 This happens because the protoplasm of the canal-cells 

 swells up and becomes converted into mucilage, which 

 exercises a pressure on the neck, and causes it to open 

 at the top, the four rows of cells being forced apart. The 

 mucilage now more than fills the canal, and forms a viscid 

 drop at the mouth of the archegonium (see Fig. 36). 



