100 



CLASSIFICATION, PHYIOGENY, AND EVOLUTION: 



the next step was the development of alternation of 

 generations. In this reproductive scheme an asexually 

 reproducing sporophyte produced spores that ger- 

 minated and then matured into a sexually reproducing 

 gametophyte. Using the plant terms sporophyte and 

 gametophyte stresses the early origin of the generalized 

 life cycle. The fertilization of gametes from different 

 gametophytes produced a zygote which differentiated 

 into a sporophyte. In the early stages of multicellu- 

 lar evolution the cellular gametophyte reached an 

 equal footing, especially insofar as size is concerned, 

 with the cellular sporophyte. Therefore, reproduction 

 soon was integrated with an increased grade of or- 

 ganization and plant origin. 



AQUATIC TO TERRESTRIAL LIFE 



Plant habitat changes from water to land most 

 likely were related to those of plant grade of organiza- 



tion and reproduction. It is here assumed that the 

 following events took place: There was increased 

 complexity in the sporophyte and simplicity in the 

 gametophyte; however, fertilization still required a 

 water medium (Figure 7.2). In the next step the 

 sporophyte developed various tissues for body support 

 and transport of nutrients. These tissues were veins 

 or vessels, structures unique to the vascular plants. In 

 addition, vascular plants became differentiated into 

 vegetative and reproductive parts, the reproductive 

 structures being cones and flowers. Once the sporo- 

 phyte became longer lived, cones and flowers could 

 form seeds in which the gamete could produce 

 another sporophyte. This step constituted the start 

 of a series of further land adaptations. Associated 

 with seeds was the carrying of sperm in wind-blown 

 pollen. This reduced the need of a water environ- 

 ment for the fertilization of eggs. Subsequent steps 

 were progressive decrease in gametophyte size and 



Ulothrix, a genus of fresh -water 

 and marine, green 

 algae 



habitat strictly 

 aquatic 



adult a gametophyte, 

 no sporophyte 



isogametes require 

 a water medium 



Botrychium, a genus of ferns found in moist 

 to somewhat dry habitats, but al 

 habitats are moist during the 

 reproductive periods 



both spores and 



heterogametes 



require a water 



medium 

 gametophyte 



sporophyte 



gametophyte 



sporophyte a 

 parasite on the 

 gametophyte 



both spores and 

 heterogametes 

 require a water 

 medium 



Mimulus, a genus of flowering plants 

 found in wet to dry habitats 



gametophyte 

 barely present 

 withm a modified 

 spore, a pollen 

 grain, or a 

 flower ovary 



reproduction does 

 require a water 

 medium 



<^ 



Anthoceros, a genus of horned liverworts 

 typically found in moist land habitats 



Figure 7.2 Evolution from aquatic to terrestrial life in plants. Certain living plants probably 

 resemble (in structure and habitat) some of the ancestors that actually accomplished the origtnol 

 transition from ocean to land. Some of these living creatures are portrayed. The water medium 

 required by hAimulus is internal or in the plant rather than external or in the environment. 



