HIGHER LAND PLANTS 



185 



two kinds of spores and are on modified leaves as in 

 ferns; gametophyte functionally dependent upon the 

 sporophyte (as in conifers), so the gametophyte and 

 its contained embryo sporophyte are parasitic upon 

 the old sporophyte, and all three generations con- 

 tribute to seed formation. 



ORDER CYCADALES (Cycads) 



Diagnosis: known from Triassic to Recent ; only 

 one United States species; generally resemble small 

 tree ferns or palms, but up to 60 feet tall; leaves typi- 

 cally palm-like, firm, spiny-tipped, and pinnately 

 compound; leaves unfolding with development and 

 resulting in a radiating crown of leaves at the apex of 

 an aerial stem; stem regularly simple and aerial, but 

 sometimes simply branched, lower stem covered by 

 stiff remnants of leaf petiole bases; some stems are 

 underground and tuberous; sporangia separated into 

 male and female cones, but no male cone in the genus 

 including the United States species; rather, sporangia 

 upon modified leaves grouped in a loose crown; cones 

 mostly at a stem apex and above an axillary bud; 

 male and female cones on separate plants (Figure 

 11.21). 



Occurrence: about 65 tropical and subtropical 

 species, one in Florida; represent the remnants of a 

 once much larger group. 



CLASS GINKGOAE (Maidenhair Tree) 



ORDER GINKGOALES 



Diagnosis: known from the Permian (indefinitely 

 from Pennsylvanian and even Upper Devonian) to 

 Recent; one living species Ginkgo biloba, native to 

 China; but probably no longer occurs in the wild; 

 commonly large trees to over 100 feet tall and with 

 bases 4 feet wide; typically consisting of a straight 

 central axis, but erect to drooping lateral branches 

 usually start from near the ground level (branches 

 approach the diameter of the central axis), sometimes 

 tending toward a basal trunk and many branches; 

 leaf blades wedge-shaped or fan-like, 1 to 2 inches 

 long, petiole somewhat longer; leaves deciduous and 

 resembling maidenhair ferns {Adiantum), mostly upon 

 short shoots and not upon the main branches; two 

 spore types, plants male or female; males with cones 

 similar to those of cycads; females without typical 

 cones, female "cone" consisting of a long stalk, stalk 



Figure 11.21 Life cycle of Zam/a, a cycod: e., female sex organ with 

 fertilized egg; f.g., female gametophyte; f.m., functional spore; i., 

 integument; m., micropyle; mes. and mis., sporangium-bearing leaves; 

 msp., male spore; n., nuceilus; p.g., pollen groin; t.m., four spores; s., 

 sporongium. (Used by permission, from Arthur Cronquist, Iniroductory 

 Botany, Horper, New York, 1961.) 



apex mostly with two ovules (one usually degener- 

 ates), base of each ovule surrounded by a "collar"; 

 mature seeds with a fleshy covering ("apricot-like") 

 (Figure 11.22). 



CLASS CONIFERAE (Conifers and allies) 



Diagnosis: known from Mississippian to Recent; 

 generally much like the Cycadae except that the 

 leaves are generally small, simple, and not bipinnate 

 (some Ginkgo leaves are divided many times); mostly 

 trees wdth stems and roots similar to the Cycadae; 

 two spore types like the Cycadae, but sporangia ap- 

 pear to be borne upon structures that never develop 

 through a leaf stage (however, even if this were true, 

 the conifer condition could represent no more than a 

 developmentaT'short cut") (Figure 1 1.23); in modern 



