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THE CONTINUITY OF THE RACE 



able environmental conditions. Two alga cells fuse to form a zygote, which 

 after a resting period will divide to form new individuals or a new colony. 

 The two cells that fuse may be alike (isogametes) or unlike (heterogametes) . 

 Isogametes may not only be indistinguishable from each other but also 

 from other vegetative cells (Fig. 17.1); more often they are small motile 

 cells formed by the division of larger vegetative mother cells. In many 

 algae the reproductive cells are heterogametes; that is, they are differen- 



FiG. 17.2. Spore production by Pilobolus, an algalike fungus (phycomycete) that aims 

 its spore-bearing organs (sporangia) and shoots the spores toward the light. The trans- 

 parent bulb at the tip of the sporangium acts as a lens, concentrating light on the side 

 opposite the light source. The light stimulates growth of that side of the filament, causing 

 it to bend. When the shadow of the dark spore falls over the base of the bulb, the whole 

 structure is aimed toward the light. Pressure gradually increases within the bulb until 

 finally it explodes, shooting the spores toward the light. (Photo by Prof. E. B. Mains.) 



tiated into small, motile, spermlike male gametes and larger, nonmotile, 

 egglike female gametes. In every instance the fusing cells are haploid, and 

 the resulting zygote is diploid. 



Except in a few of the higher algae, the diploid condition in thallophytes 

 is restricted to the zygote. At the first cell division the zygote undergoes 

 a reduction division and so produces only haploid daughter cells. 1 Here 



1 In some green algae, the first division of the zygote produces two vegetative 

 cells that continue to multiply by fission; in other species, two or more consecutive 

 divisions result in the production of small haploid spores, each capable of giving rise 

 to a new vegetative colony or individual. 



