Page Two 



E \' O L U T I O N 



July, 1929 



Are There ''Missing Links" in Plants? 



Bv RALPH H. CHENEY 



i; 



!»<,; 



Fossil Bacteria i 

 (Magnified 



THE steps by which plants evolved are made clear 

 by the similarities of structure between ancient 

 fossil plants and their living kin of today. In the 

 March" issue of EVOLUTION, Dr. Florence D. 

 Wood has already shown that the 

 relationships within the basic plant 

 groups can be definitely worked out 

 from living forms. But to trace 

 satisfactorily the descent of one 

 major group from another requires 

 the help of the fossil record, which, 

 fortunately, is quite adequate. The 

 missing links are in fact very few. 

 The anti-evolutionists often make 

 much of the apparent absence of 

 connecting links between the major 

 groups of animals, but they can- 

 not complain on that score as far 

 as the plants are concerned. For the fossil plant record 

 is singularly complete. This difference between the 

 animal and plant records can readily be explained by 

 the fact that nearly all the basic groups of animals 

 had already evolved at the opening of the Cambrian 

 period when our better preserved fossil record begins. 

 On the other hand, all but the lowest of the great 

 groups of plants have arisen later, in the time covered 

 more or less adequately by the geological record, so 

 that we can demonstrate quite clearly each transition 

 from group to group. The story of the development 

 of the four major groups of plants, one from the other, 

 is well substantiated by several parallel lines of evi- 

 dence, fossil and otherwise. 



Each living species of plant is, of course, descended 

 from older forms, but these ancestral forms may or 

 may not have survived to the present day. In view of 

 all the vicissitudes to which fossils are subject, it is 

 unreasonable to expect always to find in fossil form 

 the direct ancestors of living species. Most evolution- 

 ary series represent merely the structural stages 

 through which the plant kingdom has passed in pro- 

 ducing the more complex and modern types of plants. 

 The record is most complete for recent forms, but there 

 are also numerous fossils of the lower forms, in fact 

 a rather surprising representation when we consider 

 the almost incredible age of these forms and the 

 many destructive geologic upheavals to which they 

 have been subjected since their time. Though it must 

 be admitted that the remains of the Mosses are but 

 fragmentary, the fact of plant evolution is well estab- 

 lished by the abundance and completeness in detail of 

 the fossil records proving the definite development of 

 the Gymnosperms (cone-bearing evergreens) from 

 fern-like seed plants and of the higher flowering plants 

 from these Gymnosperms. 



The earliest forms of life must have been plant- 

 like organisms, for only plants can manufacture food- 

 stuffs out of mere mineral matter, gases and water. It 

 is to be expected, therefore, that the earliest fossils 

 would be simple plant types, and so they are. In the 

 Proterozoic deposits, from which no sure signs of ani- 

 mal life have yet come, have been found fossil bac- 



teria and Blue-green Algae (one-celled or chains of 

 cells such as pond-scum), these being also the simplest 

 plants alive today. 



Pl.inls. a- well as animals, lived only in the water 

 during the earliest stages of their 

 racial development. The Liver- 

 ^**^^'" V worts, the most simply constructed 



'*- ■ • of all Mosses, were the first plants 



■ j to develop sufficient protection 



against surface evaporation to solve 

 ,»*i ,. ■ the problem of existing on land. 



They lacked true roots or stems, 

 . ^.^ -..,,, their bodies consisting of flat, mem- 



braneous masses that formed an 

 overlapping, carpet-like mat cover- 

 ing exposed areas. This important 

 .step from water to land was prob- 

 ably taken early in the Devonian. 

 In their further progressive evolution, the plants left 

 a fossil record of achievements in structure which ac- 

 cords well with the order of development inferred 

 from the evidence of living structures. During the 



THE PLANT FOSSIL RECORD 



(Read from Bottom up) 

 (Figures show millions of years ago) 



X, 



Billion y'ears Old. 

 180 diameters ) 



