PTERIDOPHYTES 



143 



direction. In Tmesipteris a sterile plate divides the. large sporangium into two 

 chambers so distinct that they are called two sporangia 

 (figs. 326-329). That this partition is sterilized spo- 

 rogenous tissue is proved by its development and by 

 the fact that in exceptional cases it functions as spo- 

 rogenous tissue. In Psilotum the same condition occurs, 

 except that the development of .two sterile plates results, 

 in three chambers, or a*gronp of three sporangia (figs. 

 330, 331). The other noteworthy feature of Psilotum 

 and Tmesipteris is the development of the subarche- 

 sporial pad of Lycopodiales into d short stalk, which 

 bears the two or three sporangia and is called the spo- 

 rangiophore. 



(3) SPHENOPHYLLALES 



This group contains the single large Carboniferous 

 genus Sphenophyllum, which illustrates the further de- 

 velopment of the sporangiophore. In Sphenophyllum 

 there is a distinct strobilus, with whorls of linear spo- 

 rophylls coalescent at base into spreading funnels. 

 From the adaxial surface of these sporophylls sporangio- 

 phores arise, which vary from very short to very long, 

 simple or branching, and bear one to several sporangia. 

 In this genus, therefore, the sporangiophore develop- 

 ment is carried much farther than in Psilotum and 

 Tmesipteris, resulting in a multiplication of sporangia 

 by means of the sporangiophore. 



(4) EQUISETALES 



This great group is represented in our present 

 flora by the single genus Equisetum, comprising 

 about twenty-five species of horsetails or scouring 

 rushes. This is only a remnant of a great group 

 that flourished in the Paleozoic along with the 

 ancient Lycopodiales. 



Sporophyte. The sporophyte body consists 

 of a subterranean, dorsiventral main axis, which 

 gives rise to erect (radial) aerial branches, them- 

 selves simple or branched (figs. 332, 333). Equi- 

 setum is characterized by its remarkably , small 

 leaves, which for the most part are insignificant 

 scales that occur in a whorl at each joint and 

 coalesce to form a close sheath. As a conse- 



331 330 



FIGS. 330, 331. Psilo- 

 tum: 330, general habit, 

 showing the branching 

 body bearing much re- 

 duced leaves (scales) and 

 the characteristic three- 

 lobed sporangium (or 

 synangium of three spo- 

 rangia); 331, the spo- 

 rangia in greater detail. 



