SECTION 17.] 



PTERIDOPHYTES. 



157 



side and discharge a great number of green spores of a size large enough 

 to be well seen by a hand-glass. The spores are aided in their discharge 



and dissemination by four club-shaped threads attached to one part of them. 

 These are hygrometric : when moist they are rolled 

 up over the spore ; when dry they straighten, 

 and exhibit lively movements, closing over the 

 spore when breathed upon, and unrolling promptly 

 a moment after as they dry. (See Fig. 493-498.) 

 486. Ferns, or Filioes, a most attractive family 

 of plants, are very numerous and varied. In warm 

 and equable climates some rise into forest-trees, 

 with habit of Palms ; but most of them are peren- 

 nial herbs. The wood of a Fern-trunk is very dif- 

 ferent, however, from that of a palm, or of any exogenous stem either. A 

 section is represented in Fig. 500. The curved plates of wood each ter- 



FiG. 493. Upper part of a stem of a Horsetail, Equisetum sylvaticum. 494. Part 

 of the heart or spike of spore-cases, with some of the latter taken off. 495. View 

 (more enlarged) of under .side of the shield-shaped body, bearing a circle of spore- 

 cases. 496. One of the latter detached and more magnified. 497. A spore with 

 the attached arms moistened. 498. Same when dry, the arms extended. 



Fig. 499. A Tree-Fem, Dicksonia arborescens, with a yonng one near its base. 

 In front a common herbaceous Pern (Polypodium viUgare) with its creeping stem 

 or rootstock. 



Fio. 600. A section of the trunk of a Tree-Fern* 



