344 



BOTANY OF THE LIVING PLANT 



It consists when mature of a peripheral wall of cells constituting the 

 projecting neck, and a central group arranged serially. The deepest- 

 seated of these is the large ovum, which is sunk in t;he tissue of the 

 cushion ; above this is a small ventral-canal-cell, and a longer canal-cell 

 (Fig. 285, A). If prothalli be grown in moist air, and only watered by 

 absorption from below, the archegonia will have no access to fluid 

 water, and they will remain closed. Fertilisation is then impossible. 

 But if they are watered from above, as they would be by rain in the 

 ordinary course of nature, the external fluid water will bathe them, 

 and rupture will result. This may be observed in living archegonia 

 which have been kept relatively dry, and then mounted in water. 

 The neck bursts at the distal end, owing to internal mucilaginous 



K" 



FIG. 285. 



Archegonia of Polypodivm vulgare. A , still closed. o = ovurn. K' = canal-cell. #"= ventral 

 canal-cell. B an archegonium ruptured. ( x 240.) (After Strasburger.) 



swelling, and its cells diverge widely. The canal-cell and ventral- 

 canal-cell are extruded, and the ovum remains as a deeply seated 

 spherical protoplast, while access to it is gained through the open 

 channel of the neck (Fig. 285, B). Thus the same condition leads to 

 the rupture both of the male and female organs. In nature a shower 

 of rain would supply the necessary water, which would serve also as 

 the medium of transit of the spermatozoids to the ovum. But the 

 movements of the spermatozoids are not subject to blind chance. It 

 has been shown that diffusion of a very dilute soluble substance, such 

 as malic acid, into water serves as a guide, the spermatozoids moving 

 towards the centre of diffusion. Probably it is in this way that they 

 are attracted to the neck of the archegonium, which they may be seen 

 to enter, and finally one spermatozoid coalesces with the ovum (Fig. 

 286). Fertilisation is effected by entry of the male nucleus into the 

 female nucleus, and their complete fusion (Fig. 287). Thus the presence 



