286 



EVOLUTION 



Harold C. Bold 



Development of Plants in Time 



Reprinted with the permission of the author 

 and publisher from Chapter 10 of The plant 

 kingdom. Englcwood Cliffs, New Jersey, Pren- 

 tice-Hall, Inc., 1960. 



Current classifications of the plant tion in sexual reproduction are un- 



kingdom are natural or phylogenetic doubtedly responsible for changes in 



in that they attempt to group organ- individuals, populations, and species, 



isms in categories which indicate real, These are effected by natural selec- 



genetic relationships. Now that we tion. 



have surveyed representative types of From such evidence, we assume 

 plants, we can discuss the possible rela- that the operation of similar mecha- 

 tionships of currently living plant nisms over millions of years, together 

 groups to each other and to extinct with natural selection, which has re- 

 members of the plant kingdom, the suited in the extinction of individuals 

 latter known to us only as fossils, and species, have brought about the 

 Phylogenetic systems of classification, diversity now apparent in our extant 

 as indicative of real relationships of flora and fauna. No experimental evi- 

 higher taxa such as families, orders, dence is available to test relationships 

 classes, and divisions, are speculative, among plants of the past and their 

 necessarily the more so, the higher the supposed descendants now living. The 

 category. This is in contrast to evi- evidences which support speculations 

 deuces of evolutionary change in indi- regarding the origin and putative rela- 

 viduals and species in the study of tionships among the groups of plants 

 which an experimental procedure is in our present flora and those of the 

 possible. Thus, X-rays, chemicals such past reside in comparative morphology 

 as nitrogen mustard gas, and other of hving plants, their comparative bio- 

 agents produce changes or mutations chemistry, and the studv of extinct 

 in individuals which may be trans- plants are revealed by the fossil rec- 

 mitted to their offspring, a direct and ord. A few examples of these several 

 incontrovertible evidence of change lines of evidence will be presented in 

 and relationship by descent. The oc- the following paragraphs, 

 currence of such mutations in nature The comparative study of plants 

 and their segregation and recombina- (and animals) reveals certain common 



