EVOLUTION OF THE PLANT KINGDOM 415 



lings of some species of Acacia have bipinnate leaves that are soon 

 replaced by phyllodia, the adult form of foliage. In such species tran- 

 sitional stages are common. In the development of the common bracken 

 fern {Pteridium aquilinum), the young plant passes through a protostelic 

 and then a siphonostelic stage before the dictyostelic or permanent condi- 

 tion is reached. These stages represent a phylogenetic series. In the 

 seedlings of most monocotyledons the stem is at first a siphonostele, grad- 

 ually becoming a dictyostele with scattered vascular bundles. This 

 indicates that the siphonostelic condition is the more primitive one. 



PROMINENT EVOLUTIONARY TENDENCIES 



Comparative morphology furnishes abundant evidence of evolution 

 along determinate lines. Changes that have taken place in definite direc- 

 tions are apparent throughout the plant kingdom. Because some mem- 

 bers are more advanced than others, it is possible, within a group, to 

 construct a series of forms showing various degrees of modification. Such 

 a series may indicate either an advance or a decline, depending on whether 

 evolution has been progressive or retrogressive. The species of Lycopo- 

 dium display various stages in the organization of a strobilus, while the 

 genera of Fucales show a reduction series with respect to the number of 

 eggs produced in an oogonium. Often an advance in one direction has 

 been accompanied by a decline in another. The development of an 

 irregular corolla in certain families of angiosperms, such as the Labiatae 

 and Scrophulariaceae, has resulted in a reduction in the number of sta- 

 mens from five to four or two. Evolutionary tendencies, whether pro- 

 gressive or retrogressive, usually continue as long as the group displaying 

 them persists. Some of the more conspicuous evolutionary tendencies 

 seen in the major groups of green plants will be briefly summarized. 



Algae. Among the algae vegetative advance has been marked by the 

 organization of single cells into colonies and the development of multicel- 

 lular bodies into filamentous, plate-like, and massive types. Progress is 

 also shown by the beginning of cellular differentiation, resulting in spe- 

 cialization of different parts of the body for particular functions. 



From a condition where reproduction is wholly asexual, an advance is 

 seen in the establishment of sexual reproduction and in its change from 

 isogamy to heterogamy. The tendency to interpose a vegetative growth 

 phase between gametic union and meiosis has resulted in the establish- 

 ment of an alternation of haploid and diploid generations. Some algae 

 show a tendency to develop the sporophyte at the expense of the gameto- 

 phyte. The algae are a polyphyletic group representing a number of 

 parallel evolutionary lines whose connections are very uncertain. Most 

 groups seem to have arisen independently from a flagellate ancestry. 



