518 PLANT GROWTH AND PLANT COMMUNITIES 



all the others, as well as the value of such general characteristics as the 

 size, number, and arrangement of the flower produced." Other varia- 

 bles will be introduced into the situation by the economy of the plant 

 as a whole, its habitat, and its mode of life. The eight characters used 

 in taxonomy and held to be primitive in floral morphology— namely, 

 corolla present, polypetaly, actinomorphy, numerous stamens, apo- 

 carpous carpels, many ovules, axial placentation, and superior ovary- 

 may each show characteristic evolutionary advances (e.g., actinomor- 

 phy to zygomorphy, polypetaly to gamopetaly, etc.). Theoretically, 

 several or all of these morphological advances might be combined, 256 

 different combinations being possible, but actually only some 34 per 

 cent are realized. About half of these are found in only one or two 

 families (or groups), but certain combinations are present in a large 

 number of groups. 



Stebbins considers that the eight characters are not simply com- 

 bined at random in the different families and genera but are affected 

 by considerations such as constructional feasibility, functional effi- 

 ciency, and adaptation. Certain combinations occur in only a few 

 groups— e.g., the Leguminosae ( sens. lat. ) —but in them the number of 

 genera is very large. The same is true of the Orchidaceae, Gramineae, 

 Cruciferae, Malvaceae, Scrophulariaceae, Rubiaceae (in part), and 

 Compositae. Some combinations of two or three characters are es- 

 pecially prevalent— e.g., apetaly combined with a reduction in ovule 

 number, floral symmetry, and type of inflorescence (zygomorphy and 

 racemose inflorescence). In other words, the effects of correlative de- 

 velopment are abundantly exemplified in floral evolution and may ex- 

 tend to related vegetative and habitat features. Stebbins makes the 

 point that while the relative simplicity of plant structure, as compared 

 with that of animals, restricts the extent of any evolutionary progres- 

 sion, it admits of "an exceptionally large amount of parallel variation, 

 so that morphological similarity is much less indicative of phylogenetic 

 relationship in plants than it is in animals." Critical investigations of 

 some of the interesting ideas he has advanced seem feasible and desir- 

 able. The morphogenetic examination of related species which have 

 been closely investigated genetically seems likely to yield valuable in- 

 formation, especially where the action of particular genes is known to 

 be associated with conspicuous morphological developments; for, al- 

 though the effects of mutant genes are usually very slight, instances 

 are known in which the morphological changes are conspicuous and 

 extensive. 



Attention may also be called to the probable value and general 

 biological interest of morphogenetic studies of vegetative and repro- 

 ductive organs showing special adaptations. Thus we may inquire how 

 and when the special adaptation has its inception and elaboration dur- 



