502 



BOTANY OF THE LIVING PLANT 



soon show active movement, and the spermatozoid which had already 

 been formed within each of them escapes from its mucilaginous sheath, 

 and moves freely in the water by means of active cilia attached 

 near one end of its spirally coiled body (Fig. 393, 8). 



The archegonium also originates from a single superficial cell, and 

 grows out so as to project from the downward surface of the thallus. 

 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 the tissue of the 

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

 (Fig. 394, A). If prothalli be grown in moist air, and only watered 

 by absorption from below, the archegonia having no direct access 



Fig. 394. 



Archegonia of Polypodium vulgare. A, still closed. o=ovum. K' — canal-cell. AT"=ventral 

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



to liquid water 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 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 

 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. 394, 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 



