and serve as the source for future genetic 

 variation and evolutionary innovation. 



Continued genetic "trial and error" 

 eventually led to adaptations permitting 

 plants to survive longer and longer periods 

 of exposure to the air and culminating in 

 plants with cuticles, stomata, and cu- 

 tinized spore walls. This scenario also ex- 

 plains why these early, nonvascular land 

 plants are not often found in the fossil 

 record. Not only are small organisms 

 likely to be overlooked; they are also not 

 likely to be preserved, particularly in dy- 

 namically changing freshwater habitats, 

 such as those proposed here as the cradle 

 for early land plant evolution. 



When the second, rapid phase of land- 

 plant evolution began with the appearance 

 of vascular tissues, it was attended by an 

 overall evolutionary increase in plant size 

 and height. Larger plants are not only 

 more efficient at conserving water on land; 

 they also can produce more spores and el- 

 evate their reproductive and photosyn- 

 thetic organs above shorter neighboring 

 plants that are competing for the same re- 

 sources (water, sunhght, and space). 



The increase in plant size and height 

 was likely the outcome of a biological 

 arms race in which the weapons were ex- 

 tensive root systems to absorb water, a 

 canopy of leaves to intercept all the avail- 

 able sunlight, and tall, robust stems ca- 

 pable of elevating and dispersing repro- 

 ductive organs far above, and away from, 

 the interference of neighboring plants. 

 This increase in plant size required vascu- 

 lar tissues through which water and other 

 nutrients are rapidly transported from one 

 part of a plant to another. And because 

 vascular tissues are very strong and com- 

 paratively Ught in weight, their mechani- 

 cal properties are ideal for building very 

 tall structures. Indeed, many biologists 

 tend to forget that the largest organisms 

 that ever lived are trees. Built of vascular 

 tissue — wood — the largest modern se- 

 quoia is taller (longer) and more massive 

 than any whale or dinosaur! 



Although they do not have access to 

 H. G. Wells's time machine, evolutionists 

 can draw on a wonderful fossil record — 

 documenting more than three billion years 

 of biological history — for an understand- 

 ing of the mutability and adaptiveness of 

 life in response to the physical challenges 

 posed by constantly changing environ- 

 ments. They also benefit from knowing 

 that the outcome of evolutionary experi- 

 ments must comply with the laws of 

 physics and chemistry, as well as the rules 

 of geometry. 



A boat brings you as close 

 as any human is allowed to the Bird Islands. 

 Watch for cormorants, razorbills, petrels and terns 

 as well as the rare Atlantic Pujfin. 



You'U find thousands of exciting things to do in our 1994 Travel Guide. 



And with an average 30% exchange rate your money goes further here. 



For your FREE Guide plus our Value- Vacation Catalogue, call: 



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