EVOLUTION OF THE PLANT KINGDOM 413 



indicates from which older group a younger one has been derived. Inter- 

 relationships among the larger groups must be inferred principally from 

 evidence based on studies in comparative morphology. The "lower" 

 groups of plants are merely those which have undergone relatively little 

 modification, the "higher" groups, a vastly greater amount. Existing 

 groups are usually represented as twigs on a phylogenetic tree. The 

 larger branches denote divergent lines of descent. These are often 

 obscure and difficult to trace, but become clear as knowledge advances. 



Specialization. The general trend of evolution toward greater adapta- 

 tion to the environment has resulted in the development of specialized 

 forms from generalized forms. Structural complexity is ahvays a result 

 of evolution from a simpler condition of organization, but simplicity does 

 not always represent a primitive state. Often it denotes reduction from 

 a more highly developed ancestry. Sometimes there is structural evi- 

 dence of such reduction, especially in ontogeny, but usually this evidence 

 is obscure or wanting. Consequently it is often difficult to ascertain 

 whether structural simplicity is a primary or a derived condition. 



Higher types have arisen from generalized members of lower groups, 

 not from speciahzed members. Highly specialized groups, like the red 

 algae and the mosses, represent blindly ending lines of descent. They 

 may change in the direction of greater specialization, but cannot revert to 

 a generalized condition and then become specialized in another direction. 



In many groups evolutionary advance has not affected all parts of the 

 plant to the same extent and, as a consequence, advanced features are 

 often combined with primitive ones. For example, although the cycads 

 are seed plants, they have retained swimming sperms, an extremely 

 ancient character, the phylogenetic continuity of which can be traced back 

 to the algae. Sometimes the development of one character is associated 

 with the suppression of another. Thus, in the Compositae, the formation 

 of an involucre has resulted in a reduction of the calyces of the individual 

 flowers in the head. Similarly the strong development of mechanical tis- 

 sues in the stems of many large herbaceous angiosperms is related to the 

 weak development of xylem and might be regarded as a compensation 

 for it. Such instances of compensation are common throughout the 

 plant kingdom. 



Parallel Development. The same evolutionary tendency, acting inde- 

 pendently in different groups of plants, may bring about similar changes, 

 thus resulting in parallel development or homoplasy. For example, 

 heterogamy has arisen independently in a number of widely separated 

 algal groups and heterospory in various groups of vascular plants. Epig- 

 yny has developed independently in many different families of angio- 

 sperms. Structural similarity resulting from parallel development is no 

 indication of phylogenetic relationship. It merely signifies that evolution 



