Evolution of Plants 351 



prothallus on which the sexual organs arise, and which still 

 is, or once was, an independent plant. All however develop 

 antheridia and archegonia, these arise in a fundamentally 

 similar mannei in all, and they all have polyciliate sperms, 

 that in number of cilia show considerable advance on the four 

 cilia of Chsetophoracese. 



If now we attempt to trace the probable origin of the Equi- 

 setales or great horsetail alliance, by combining palseonto- 

 logical and morphological evidence in an effort to fill the gap 

 between them and the green algse, it may be considered that 

 a group early evolved in which the fertilized egg was borne 

 on a semi-aquatic to subaerial prothallus. Such prothallia 

 were probably produced during a moist period of the year 

 and may then have become exposed to a rather suddenly 

 changing subaerial xerophytic state, that caused retardation 

 in growth of the first two or three leaves of the young embryo 

 and arrest of the associated internodes, with resulting reduc- 

 tion in size and whorled placing of the leaves. So the forma- 

 tion of two to three leaves from a common foliar ring on the 

 embryo plantlet, followed by whorls of leaves of increasing 

 number, must have occurred. It seems not unlikely that 

 such may have arisen over some land area exposed to hot 

 suns, and in which periods of alternate flooding and drying 

 of low flat lands took place. 



Thus must have originated, during the late silurian or early 

 devonian period, types like the Sphenophyllege, that are the 

 most ancient and most primitive in structure of the great 

 horsetail alliance or Equisetales. 



In succession to these the Archseocalamitese and the Calam- 

 itese appeared, from the middle devonian onward, and attained 

 maximum development during the later carboniferous period, 

 alike in size, in structure, in abundance, and in wide distribu- 

 tion. The highest members of the series like C alamo stachys 

 advanced to the stage of bearing microspores and megaspores 

 in adjoining sporangial sacs, while all of the Calamiteae had a 

 capacity for indefinite stem increase, that is not shown in 

 their nearest existing representatives, the horsetails. Mean- 



