HIGHER LAND PLANTS 



173 



Figure 11.5 The two living psilopsid genera, Tmesipterus (left, 

 slightly less than natural size) and Piilotum (center, slightly less than 

 natural size, and right, enlarged sporangia). (Used by permission, from 

 Arthur Cronquist, Introductory Botany, Harper, New York, 1 961 .) 



the sporophyte rhizome; sex organs develop from 

 single cells, male organs are globular to hemi- 

 spherical and female organs, are necked and flask- 

 like; gametophyte aerial or subterranean but 

 usually brown and without chlorophyll, nutrition 

 involving a fungus (Phycomycete). 



Reproduction : flagellate sperm swim through water 

 from a male sex organ into the female organ neck and 

 to the egg; mitotic division of the fertilized egg (zy- 

 gote) takes place and the embryo soon differentiates 

 into a bottom foot and an upper shoot; the shoot de- 

 velops into the mature sporophyte; when the shoot 

 functions independently, a layer of cells forms be- 

 tween the shoot and foot; such layers in any plant are 

 for the separation of parts; in psilopsids, foot and 

 shoot are separated; after some time the gametophyte 

 dies and decays (Figure 1 1.6). 



SPORANGIUM 



Figure 11.6 tife cycle of Psitotum, the whisk fern: e., egg; e.s., 

 embryo sporophyte; s., sperm; z., zygote. (Used by permission, from 

 Arthur Cronquist, Introductory Botany, fHorper, New York, 1961.) 



SPOROPHYTE 



Structure: fossils to about 10 feet tall, living forms 

 to about 2 feet tall; herbaceous perennials; with 

 stems, including underground rhizomes, and leaves 

 (leaves well developed in only one of the three living 

 species), but without roots; sporangia mostly on 

 branch ends, terminating ordinary branches in most 

 fossils but on short specialized branches in living 

 forms; leaves at the base of minute sporangia 

 branches in living forms cause sporangia to appear to 

 be in the axils of leaves. 



Sporangium : a two- or three-lobed (three-lobed is 

 the only species in the United States), semiglobular 

 structure; if truly lobed, sporangia represent the fu- 

 sion of two or more individual sporangia (see Marat- 

 tiales in the Class Filicineae) and each sporangial 

 lobe is essentially a single sporangium; all spores of 

 the same size and shape; in known fossil forms the 

 sporangia are definitely single and range from little 

 more than a fertile portion of a terminal shoot to a 

 definite sporangium. 



Stems: branched, green, and photosynthetic in liv- 

 ing species; in all species branching is mostly equal 

 forking; rhizome only slightly differentiated at most; 

 often turned upward and gives rise to regular stems. 



Leaves: either present and alternate upon stem, 

 restricted to the base of the shoot or to the vicinity of 



