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A TEXTBOOK OF THEORETICAL BOTANY 



foot it develops rhizoids externally and a vascular stele and mycorrhizal cells 

 internally, and thus begins a new rhizome, which continues to branch either 

 dichotomously or by the formation of adventitious side branches. 



Endodermis 



Endophytic 

 Fungus 



Vascular 

 strand 



Antheridium 



Archegonium 



Fig. 627. — Psilotum triqiietnim. Sections of the prothallus 

 showing development of vascular tissue. {After HoUoivay.) 



Psilotum possesses a collection of primitive characters without a parallel 

 among living Pteridophyta and clearly indicating a relationship to the ancient 

 Psilophytales. They may be summarized as follows : — 



The predominance of dichotomy. 

 The absence of roots. 

 Protostelic structure of the rhizome. 

 Mesarch sporangiophore traces. 

 Terminal sporangia. 



6. Homospory. 



7. Thick sporangial walls. 



8. Close similarity of both generations. 



This last point is not without significance in regard to the origin of the 

 alternation of generations, when we consider the primitive status of the plant, 

 and its relationship to one of the most ancient groups of Vascular Cryptogams. 



I. 

 2. 



3- 

 4- 

 5- 



