CHAPTER XXIX. 



INTRODUCTORY TO LAND- VEGETATION. 



The Thallophyta are essentially aquatic in their characters. Most 

 of the Algae are actually water-living organisms : and though some 

 of them exist on land-surfaces, they are restricted as a rule to moist 

 situations, and none of those now living have achieved a dominant 

 position in subaerial vegetation. In the last paragraphs of Chapter 

 XXVII. (p. 444) it has been shown how Fungi, though referable in 

 origin to Algal sources, and showing evidence of it particularly in 

 the Phycomycetes, have in many of their characters become specialised 

 so as to accommodate them to subaerial life. Even extreme examples 

 of this mode of life are still traceable by comparison to an Algal 

 origin. The conclusion may then be held as justified for the Thallo- 

 phyta generally that they originated in water. 



We pass now to Plants which have acquired a firmer hold on 

 exposed Land-Surfaces, though still retaining features which show 

 that they were ultimately of aquatic origin. They may be held as 

 the Amphibians of the Vegetable Kingdom, being specialised in their 

 vegetative structure to subaerial life, but still retaining certain 

 features which indicate their dependence on external liquid water. 

 The most striking of these is that their fertilisation is carried out by 

 spermatozoids motile in water (zoidiogamic) . The life-cycle cannot 

 be completed without this stage, which is clearly reminiscent of their 

 flagellate origin. The Plants in question are sometimes styled the 

 Archegoniatae, a term which comprises the relatively simpler 

 Mosses (Bryophyta) and the more complex Ferns (Pteridophyta). 

 The feature which they have in common justifying that inclusive 

 name is the female organ, or Archegonium (Fig. 350). This is a 

 more or less flask-shaped body enclosing and protecting a single 

 ovum or egg, which is thus enveloped in the tissue of the parent. 



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