224 PTERIDOPHYTA. 



The elaters are extremely hygroscopic, coiling round the spore 

 when moistened, and expanding as soon as dry, presenting a most 

 lively object under the microscope when breathed upon and allowed 

 to drv. The second layer, when germination commences, becomes 

 detached from the inner wall, which is formed of the exospore 

 and endospore. 



The order has become much reduced, and at the present time 

 includes only one genus, Equi'setnm, with about twenty-five species, 

 which are distributed over the entire globe, particularly in damp 

 situations. In SOME SPECIES the barren shoots are green and very 

 much branched, but the fertile ones are unbranched, pale brown, 

 and possess no chlorophyll (E. arvense, Field-Horsetail, Fig. 224, 

 and E. maximum}. IN OTHERS the fertile and barren shoots are 

 alike green, and either both unbranched (E. luemale'), or branched 

 (E. palustre, E. limosum, etc). The fertile shoots of E, silvaticum, 



A 



FIG. 225. -Spores of Equisctum : A damp, with elaters (c) coiled round the spore ; B dry, 



with elaters expanded. 



up to maturity, resemble those without chylorophyll of E. arvense, 

 but after that period they produce green branches, and thus re- 

 semble the barren ones. 



EXTINCT ISOSPOROUS EQUISETIN.E. In addition to several true species of 

 fossilized Equisetums, the order of the CALAMITES, which no doubt is closely 

 allied to the Equisetinae, is also found in the fossil state. These were gigantic 

 forms, attaining about twenty times the size of those of the present day, and 

 stems of nearly 10-12 metres in height are known. They reached the culmina- 

 ting point of their development in the Carboniferous peiiod, and died out 

 towards the close of the Palaeozoic. The stems had hollow internodes and 

 a ternating grooves, similar to their relatives of the present day. The leaves 

 must either have been absent or very perishable, since they have not been 

 identified with certainty. If the determinations of certain remains of cones 

 which of late have been discovered are correct, they were heterosporous and 

 had two kinds of sporangia as in Ihe following sub-class. A cambium forma- 

 tion and an increase in thickness has been found in the stems. 



Their VSES are very limited. A few species, such as E. hirmale are used for 

 polishing on account of the hard siliceous cell-walls of the epidermis, found in 

 all species of Etjuisetum. 



