184 



PHYLUM TRACHEOPHYTA: 



mitosis" 



SEEDLING (2n) 



mitosis 



EMBRYO (2n) IN SEED 



mitosis 



ik 



ZYGOTE (2n) 



-►ADULT SPOROPHYTE (2n) 



MALE CONE (2n) FEMALE CONE (2n) 



\ 



SPORANGIA 

 SPORE-FORMING 

 CELLS (2n) 



meiosis 



MALE SPORES (n) 



GAMETOPHYTE 

 FORMS WITHIN 

 SPORE (pollen) (n) 



I 



pollination 



I 



pollen tube 

 growth (n) 



I 



SPERM (n) 



I 



SPORANGIA (ovules) 



SPORE-FORMING 

 CELLS (2n) 



i 



meiosis 



I 



1 FEMALE SPORE 

 PER OVULE (n) 



pollination 



I 



gametophyte 

 matures (n) 



EGG (n) 



fertilization 



Figure 1 1 .20 Spermatophyte life cycle, generalized as found in gymnosperms. 



are distinguished from flowering plants by the ab- 

 sence of complex flowers (but flower-like structures 

 are present in some), by pollen being transported 

 strictly by the wind and only accidentally by organ- 

 isms or water, by the pollen tube being inserted di- 

 rectly into the micropyle rather than having to grow 

 through tissues, by a single fertilization involving a 

 sperm and an egg rather than by double fertilization 

 in which the second process involves formation of a 

 seed nutrient (endosperm), and by seeds being 

 formed naked rather than enclosed in a "vessel." 



"Gymnospermae" is an artificial taxon in that con- 

 trasting gymnosperms with angiosperms implies 

 equivalent taxa. Actually it appears most likely that 

 the gymnosperms arose first, and one gymnosperm 



group later gave rise to the angiosperms. Also, the 

 gymnosperms appear to represent three major lines 

 of development (three classes) and the angiosperms 

 one line of development from the gymnosperms. 



CLASS CYCADAE (Cycads and fossil allies) 



Diagnosis: known from early Mississippian (indef- 

 initely from Upper Devonian) to Recent; plants gen- 

 erally fern-like to palm-like; leaves mostly large, un- 

 folding with development and pinnatified (because of 

 definite fern appearance leaves are often called 

 fronds); stems mostly above the ground or visible at 

 the surface, sometimes an underground rhizome; 

 stem unbranched or branched; sporangia produce 



