INTRODUCTORY TO LAND-VEGETATION 



455 



The ovum of Land Plants cannot, as in Fucus and other aquatic 

 plants, be safely extruded from the parent to fend for itself. An 

 unprotected primordial cell would have a poor chance of surviv.il 

 if exposed to the drying influence of the air. The ova of the 



Fig. 350. 

 Archegonia of Polvpodium vulgare. A , still closed. o = ovum. K' = canal-cell, tf "^ventral 

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



Archegoniatae are accordingly retained within the tissue of the parent, 

 and are produced singly. The archegonium is in fact a nursing organ, 

 and the constancy of its occurrence, with only minor differences of 

 detail in the Bryophyta and Pteridophyta, may be taken as evidence 

 how essential to survival of subaerial plants is the protection and 

 nursing of the germ. It may be said generally for subaerial Animals 



REDUCTION. 



5YNGAMY. 



and Plants that an internal embryology in one form or another is an 

 essential factor of life upon exposed land-surfaces. It is this which 

 follows on fertilisation of the ovum of the Archegoniatae so safely 

 hidden within the archegonium. 



