536 A TEXTBOOK OF THEORETICAL BOTANY 



Relationships of Equisetum 



The relationship of Equisetum to Calamites and SphenophyUum has been 

 mentioned above. Both these ancient types, however, had cones with leafy 

 sporophylls, and the question has been much debated whether the leafless 

 cone of Equisetum is a primitive condition or not. Anatomically there is no 

 trace of any sporophylls having formerly existed, and the opinion has gained 

 ground that we have in this genus a point of contact with a very ancient form 

 of organization. 



Briefly this view, as put forward by Zimmermann, is based upon the 

 existence in the most primitive plants of primary branches of a very simple 

 type, which he calls telomes. Some of these bore terminal sporangia and 

 were thus fertile, others were sterile, but both were morphologically equivalent. 

 This structure is actually represented by the Devonian fossil Psilopliyton 

 (see Fig. 644). From this may be derived a condition, shown by Astero- 

 calamites, in which the two types of telome were in whorls, mixed together. 

 By another line of development we get the Devonian fossil Hyenia (Fig. 645), 

 in which the fertile telomes were collected together on the upper parts of 

 certain branches, and the sterile telomes on the lower parts. From this 

 condition Zimmermann suggests that Equisetum has been evolved, while in 

 the Calamites fertile and sterile whorls alternated on the upper parts of the 

 stems which became finally difl^erentiated as cones. 



It has been customary to refer to the cone appendages in Equisetum as 

 sporangiophores, but this is purely a descriptive term, while the use of 

 the term telome connects them definitely with those primitive thallus lobes 

 from which both branches and leaves may have been divergently evolved. 



