One Giant 



Step for 



Life 



Simple, law-abiding plants led the 

 invasion of hostile lands 



by Karl J. Niklas 



I:^ In H. G. Wells's 1 895 tale The Time Ma- 

 ^ chine, a scientist travels into the future to 

 pL^ a near-lifeless earth slowly circling a 

 dying sun and finds "intensely green vege- 

 tation... the same rich green that one sees 

 on forest moss or on the lichens in caves: 

 plants which... grow in a perpetual twi- 

 light." Ironically, this melancholy descrip- 

 tion of life's closure adequately describes 

 the earth when life first colonized the land 

 some 440 million years ago. Plants were 

 the very first forms of life to migrate onto 

 land, and by providing food and creating a 

 more humid and sheltered environment, 

 they paved the way for the later coloniza- 

 tion of land by animal life. 



Although all life began in the oceans, 

 the first land plants came from freshwater 

 environments. The transition from water 

 to land was long and complex and one of 

 the greatest adaptive events in the history 

 of life. The fossil record shows that the 



transition involved two phases that collec- 

 tively lasted about 75 million years. 



The first phase got under way about 

 439 million years ago, when compara- 

 tively small and structurally simple plants, 

 resembling today's algae, began to colo- 

 nize the land. During this time numerous 

 adaptations evolved. Among the most im- 

 portant was the capacity to produce a cu- 

 ticle, a layer of waxlike material coating 

 the extemal surface of the plant body. The 

 cuticle is not required for life in water, but 

 it is the sine qua non of a land plant. Pores, 

 or stomata, in the cuticle were another es- 

 sential development, since plants need at- 

 mospheric gases for respiration and photo- 

 synthesis. (Neither nature nor the best 

 chemists have invented a material that is 

 both permeable to oxygen and carbon 

 dioxide yet impermeable to water.) Flank- 

 ing the stomata on most land plants are 

 highly specialized cells that can change 

 their size and shape depending upon the 

 availability of water. By regulating the di- 

 ameter of stomata, they can control the 

 rate at which water vapor is lost from plant 

 tissues to the air. The oldest currently 

 known fossil land plants with cuticles, 

 stomata, and guard cells are from very an- 

 cient rocks dating as far back as 410 mil- 

 lion years. Another important adaptation 

 was the evolution of plant spores with cu- 

 tinized walls that reduced water loss and 

 afforded mechanical protection as well. 



The second phase of land-plant evolu- 

 tion started about 410 million years ago 

 with the appearance of larger, more com- 

 plex plants with tissues made up of cells 



Requirements for Leaving the Water: For plants, life in 

 air first demanded cuticle, stomata, and — as they 

 became larger — vascular tissue. 



that conduct water and sap throughout the 

 plant body. Vascular tissues are the 

 anatomical hallmark of the majority of the 

 plants most familiar to us — ferns, pine 

 trees, and the flowering plants. 



All these modem plants, no matter how 

 complex, trace their evolutionary history 

 to the very first vascular plants, and their 

 diversity is the consequence of a remark- 

 ably rapid evolutionary specialization. 

 Within only fifty million years, or approx- 

 imately 12 percent of the entire history of 

 vascular plant evolution, virtually every 

 major plant group currently represented in 

 modem world floras evolved. 



This great taxonomic explosion, rival- 

 ing that of the Cambrian explosion of ani- 

 mals {see "Life's Expanding Realm," page 

 14), occurred during the Devonian. Flow- 

 ering plants, which dominate today's 

 world floras, had not yet appeared by the 

 end of the Devonian, about 360 million 

 years ago. Comparative latecomers, they 

 made their first appearance in the fossil 

 record only 125 million years ago, during 

 the Cretaceous period. 



That the initial colonization of the land 

 by plants took longer than the subsequent 

 radiation of vascular plants is not surpris- 

 ing. In many ways, life on land presented 

 huge difficulties for aquatic organisms. It 

 meant giving up unlimited access to water, 

 essential for the growth and reproduction 

 of every type of organism. It also meant 

 coping with the compressive effects of 

 gravity on body mass. (Water is roughly a 

 thousand times denser than air and affords 

 aquatic plants and animals a "mechanical 

 cushion" against the force of gravity.) In- 

 deed, we may well wonder why the land's 

 surface was colonized at all. Although we 

 may never know the answer to this ques- 

 tion, applying a little physics and chem- 

 istry provides some clues. 



Two simple facts tell us that plants had 

 something to gain by leaving the water. 

 First, water absorbs and attenuates sun- 

 light, upon which all plant fife depends. 

 Second, the need for carbon dioxide and 

 oxygen — the basic metabolic require- 

 ments of plants — is better met on land 

 than in freshwater. 



A basic law of physics — Bouguer's 

 law — shows that the intensity of Ught de- 

 creases exponentially as light passes 

 through a column of water. That is, if 50 

 percent of the available light energy is ab- 

 sorbed by the first centimeter of water, 

 then it is weakened yet another 50 percent 

 by the second centimeter, and so forth. 

 Also, the quality of light changes as it pen- 

 etrates the water column. Because wave- 



Diagrams; Karl Niklas and Joe LeMonnier 



22 Natural History 6/94 



