^ 304 PLANT MORPHOLOGY 



absent. Definite strobili usually present. Sporangia borne on sporo- 

 phylls, solitary, adaxial, unilocular; trabeculae sometimes present. 

 Ilomo'sporous (in Lycopodiales) or heterosporous. Prothallia in homos- 

 porous forms tuberous, wholly or in part subterranean, and with chloro- 

 phyll only in the aerial portion. Sperms biciliate or (in Isoetales) multi- 

 eiliate. Embryo with or (in Isoetales) without a suspensor. 



Equisetinae. Leaves mostly small and simple, numerous, cyclic, with 

 a single vein. Stems conspicuously jointed, longitudinally grooved. 

 Stem an endarch siphonostele or (in Sphenophyllales) an exarch proto- 

 stele. Leaf gaps absent. Strobili present. Sporangia generally borne 

 in groups on sporangiophores, usually adaxial with reference to the sporo- 

 phylls, which are freciuently absent. Homosporous or heterosporous. 

 Prothallia (in Equisetales) flat, aerial, green. Sperms multiciliate. 

 Embryo without a suspensor. 



Filicinae. Leaves mostly few, large, and divided, spiral, with numer- 

 ous veins. Stem a protostele, siphonostele, or dictyostele; typically mes- 

 arch. Leaf gaps present. Strobili absent. Sporangia numerous and 

 generally abaxial on modified or unmodified leaflets, commonly in sori. 

 Homosporous or (in Hydropteridales) heterosporous. Prothallia in 

 homosporous forms flat, green, and aerial or (in Ophioglossales) tuberous, 

 subterranean, and not green. Sperms multiciliate. Embryo nearly 

 always without a suspensor. 



GENERAL CONCLUSIONS 



All pteridophytes have archegonia and multicellular antheridia, 

 although these organs are somewhat reduced as compared with those of 

 bryophytes. Fertilization is still conditioned by the presence of water. 

 All pteridophytes display a distinct alternation of generations, but 

 advance far beyond the bryophytes in the possession of an independent 

 sporophyte with a leafy stem, true roots, and a well-developed vascular 

 system. The sporophyte is nourished by the gametophyte only during 

 the early stages of its development. 



Independent Sporophyte. In the evolution of the plant kingdom the 

 first land plants to have established an independent sporophyte must 

 necessarily have developed one with (1) a means of anchorage and of 

 absorbing water directly from the soil and (2) a means of displaying green 

 tissue to the light and air. In practically all existing pteridophytes the 

 sporophyte carries on water absorption by roots and photosynthesis by 

 leaves borne on a stem. Food manufacture has become primarily a func- 

 tion of the sporophyte, leaving fertilization as the main function of the 

 gametophyte. This arrangement permits the sporophyte to grow upward 

 into the air. A large plant displaying leaves to the light and air requires 

 a constant supply of water as well as a means of mechanical support. 



