EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 135 



and appear to be represented by a very few allies living today. Of 

 these the best known is Psilotiim (Fig. 30, C), which is widespread 

 in warm regions. Both extinct and living members are branched, 

 with naked or spiny stems having cuticle and stomata on the surface, 

 and sometimes small simple leaves. They form spores of one size, 

 produced in characteristic sporangia. Further details regarding the 

 Psilophytineae and also some other groups of ancient vascular plants 

 of often obscure relationship, may be obtained from such works on 

 fossil botany as are cited at the end of this chapter, though com- 

 parisons will show how difficult it sometimes is for authorities to 

 agree. Thus the present group are sometimes given the status of 

 a division, as the Psilophyta (or Psilopsida). 



The Equisetineae (Horsetails) have also a very ancient history, the 

 present-day representatives, already dealt with in Chapter II, being 

 mere depauperated relics of a once large and important group that 

 flourished at least as far back as the Devonian. With dieir extinct 

 fossil representatives they are sometimes given the rank of a division, 

 under the name of Arthrophvta (or Sphenopsida). They show 

 plentiful adventitious roots and small whorled leaves, and sometimes 

 secondary thickening. Of this major group there are five subsidiary 

 groups or orders, of which the most important are : (i) the lowly 

 Sphenophyllales, which had slender reclining stems and expanded, 

 wedge-shaped or lacerate leaves usually less than 2 cm. in length ; 

 (2) the Calamitales, which were like giant Horsetails, attaining heights 

 of some tens of feet and with their jointed, hollow stems sometimes 

 exceeding 20 cm. in diameter ; and (3) the Equisetales, which were 

 characterized by their slender, jointed stems and were altogether very 

 like the representatives living today. Although the Equisetales 

 appeared only in the Carboniferous, they were and are closely allied 

 to the earlier Calamitales, and consequently Equisetiim may be 

 regarded as the oldest living type of vascular plant ; indeed some 

 authorities maintain the Calamites and Equiseta in the same order. 

 Probably both groups arose from some common source, the 

 Equisetales lingering on to the present day without major changes 

 and representing the end of a once virile line whose ecological 

 aggressiveness still saves it from extinction. It should be noted 

 that in some Calamitales there was diflterentiation into large mega- 

 spores and small microspores. A living Equisetum is shown in 

 Fig. 15, and in Fig. 31 may be seen fossils of typical members of 

 the other two main groups. 



Living representatives of the Lycopodineae (Club-mosses and 



