CHAPTER XX. 



SUMMARY OF THE WORKING HYPOTHESIS. 





 IT will be useful to collect the substance of the preceding chapters into a 



more concise form, hypothetical and uncertain as in their very nature 

 any conclusions must necessarily be. 



The general problem of the origin of a Land-Flora is not to be solved 

 by mere observation of the present-day distribution of the organisms 

 composing it; some other basis for an opinion must be sought. The 

 problem has been approached primarily from the point of view of the 

 individual life; and special regard has been given to the relation which 

 subsists between the environment and fertilisation, the most critical incident 

 in the life of any organism (Introduction). 



It seems probable that certain Algae represent in their general characters 

 the original source from which the Land-Flora sprang. Their prevalent 

 method of fertilisation by motile gametes is by many held to show a 

 reminiscence of their ultimate origin from the free-living Flagellates : however 

 this may be, the gamete motile in water is a character which many Algae 

 share with the Archegoniatae ; it is a feature essentially typical of aquatic 

 vegetation. 



In respect of their whole life-cycle the Archegoniatae may be said to 

 show an amphibial existence, the aquatic and the terrestrial characters 

 being reflected in its two alternating phases (Chapters II. and III.). The 

 gametophyte is as a rule delicate in texture, without intercellular spaces 

 in its tissues, or a fully developed water-conducting system, while its sexual 

 organs only become functional on their rupture in water outside the 

 plant-body : the gametophyte thus proclaims its ultimate dependence on 

 external fluid water as thoroughly as an Alga. The sporophyte, on the 

 other hand, is a characteristically subaerial body : this is shown by its 

 more robust habit, its effective ventilating system, and its vascular strands 

 for the conducting function seen in all the higher forms : its final result, 

 the maturing and dissemination of spores, is normally carried out under 

 circumstances of dryness. All these features mark it as an essentially 

 terrestrial phase. 



