l6 THE MORPHOLOGY OF PTERIDOPHYTES 



be quite small that the particular microspore will have come 

 from the same parent sporophyte as the megaspore and thus 

 a fair degree of out-breeding will have been achieved. The 

 relative emancipation from the aquatic environment pro- 

 vided by the heterosporous habit will confer on the sporo- 

 phyte the freedom to grow almost anywhere that its own 

 potentiahties allow and the possibiUty of out-breeding will 

 favour more rapid evolution of those potentiahties. Most 

 morphologists agree that the evolution of heterospory was a 

 necessary step in the evolution of the seed habit and that, 

 therefore, it is one of the most important advances in the 

 whole story of land plant evolution. 



The Ufe-cycle of a typical heterosporous pteridophyte may 

 be represented diagrammatically as in Fig. 2. 



The distinction between heterospory and homospory is 

 one of the criteria used in the classification of pteridophytes, 

 in accordance with the general behef that reproductive 

 organs are a better guide to phylogenetic relationships than 

 are vegetative organs. They are held to be more *con- 

 servative', in being less susceptible to the immediate influence 

 of the environment. Likewise, therefore, the manner in 

 which the sporangia develop and the way in which they 

 are borne on the sporophyte constitute important taxo- 

 nomic characters. 



The sporangium, in all pteridophytes, is initiated by the 

 laying down of a cross-wall in a superficial cell, or group of 

 cells. Since this wall is pericUnal (i.e. parallel to the surface) 

 each initial cell is divided into an outer and an inner daughter 

 cell. If the sporogenous tissue is derived from the inner 

 daughter cell, the sporangium is described as *eusporangiate' 

 and, if from the outer, as *leptosporangiate'. This definition 

 of the two types of sporangium is usually expanded to in- 

 clude a number of other differences. Thus, in leptosporangi- 

 ate forms, the sporangium wall and the stalk, as well as the 

 spores, are derived from the outer daughter cell, but, in 

 eusporangiate forms, adjacent cells may become involved 



