INTRODUCTION 



17 



in the formation of part of the sporangium wall and the stalk 

 (if any). Furthermore, the sporangium is large and massive 

 in eusporangiate forms, the wall is several cells thick and the 

 spore content is high, whereas, in leptosporangiate forms, the 

 sporangium is small, the wall is only one cell thick and the 

 spore content is low. Of these two types, the eusporangiate 

 is primitive and the leptosporangiate advanced. 



Eggf " ) (^ Spermatozoid 



Zygote 



5 Gametophyte 



cf Gametophyte 



Meiosis 



2n 



I^^@ Microspores 



-H — ► — —(^ 







*~~vIL//'~^ Megaspores 



B 



© 



Sporophyte Meiosis 



Fig. 2 

 Life cycle of a heterosporous pteridophyte 



Until the early years of this century, it was widely beheved 

 that sporangia could be borne only on leaves and that such 

 fertile leaves, known as 'sporophylls', were an essential part 

 of all sporophytes. However, the discovery of Devonian 

 pteridophytes that were completely without leaves of any 

 kind, fertile or sterile, has led most morphologists to 

 abandon this 'sporophyll theory'. It is now accepted that in 

 some groups sporangia may be borne on stems, either 

 associated or not with leaves, and in others actually on the 

 leaves. 



Important as reproductive organs are in classification, 



