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 
_suibsists 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. 
