414 PLANT MORPHOLOGY 



in two or more different groups has occurred in the same general direction 

 and often in response to the same influence. Instances of parallel devel- 

 opment are numerous. Unless they are recognized, false conclusions 

 regarding relationships may be drawn. 



Real relationships among different kinds of plants are shown by the 

 presence of similar characters derived from a common ancestry. The 

 members of every natural group, despite superficial differences, are built 

 according to the same basic pattern. This is expressed by the characters 

 that distinguish it from other groups. Related plants display many 

 structural resemblances because of a common origin, while their differ- 

 ences are a result of divergent evolutionary tendencies. The greater the 

 degree of basic resemblance between any two kinds of plants, the closer 

 is their relationship and the less remote their common ancestry. 



Homologous Structures. In any natural group the various members 

 possess certain structures that are considered as homologous, or morpho- 

 logically equivalent. Such structures may display considerable diversity 

 in form or function, but have a similar ontogeny and so bear the same 

 relation to the plant as a whole. In liverworts, for example, spores and 

 elaters are homologous, since they develop from the same mass of undiffer- 

 entiated sporogenous tissue. In angiosperms stamens and carpels are 

 homologous with leaves, as well as with the sporophylls of other vascular 

 plants. Tendrils and thorns may be homologous either with stems or 

 leaves, depending on their place of origin on the plant. Cladophylls are 

 homologous with stems, while bracts and scales are homologous with 

 leaves. 



Recapitulation. Developmental stages not only reveal homology 

 between different kinds of structures, but frequently furnish other evi- 

 dence of evolutionary changes. In the early development of many 

 plants, stages appear that correspond to adult stages in less highly spe- 

 cialized plants. This suggests that such developmental stages may repre- 

 sent ancestral conditions. The theory that "ontogeny recapitulates 

 phylogeny" cannot be regarded as a principle of broad application, how- 

 ever, for in many plants embryonic or juvenile stages have no apparent 

 evolutionary significance. On the other hand, the theory receives sup- 

 port from many sources. For example, among kelps Laminaria is a gen- 

 eralized type, the simplest species consisting of a holdfast, stipe, and an 

 undivided blade. Most of the other kelps are more highly differentiated 

 when mature, but in early development pass through a Lamiwana-like 

 stage. This indicates that Laminaria represents the ancestral condition. 



The occurrence of a protonemal stage in the life history of a moss recalls 

 an algal stage in the ancestry. The presence of needle-like leaves on the 

 seedlings of certain conifers and of scale-like leaves on older plants sug- 

 gests that the juvenile foliage represents the ancestral type. The seed- 



