212 THE FLOWER 



placenta. Which suture of the pod is this (Sec. 98)? 

 Compare with your sketches of dehiscent 

 fruits ; which one does it resemble ? 



Examine a bud and diagram the aesti- 

 vation. Which petal overlaps the others? 

 Diagram the flower in horizontal and ver- 

 papiiionaceous tical section, and decide upon the following 



points : - 



What is the numerical plan ? 



In what organ or organs is there a departure from 

 symmetry ? 



In which is there irregularity ? 



Are all the parts free ? 



In which set of organs is there union ? 



Is the flower hypogynous or epigynous ? 



301. Significance of these Distinctions. These distinc- 

 tions are important to remember not only because they are 

 very useful in grouping and classifying plants, but because 

 they mark successive stages in the evolution of the flower. 

 In general, flowers of a primitive type and less advanced 

 organization are characterized by having their organs free 

 and hypogynous, while the more highly developed forms 

 show a tendency to consolidation and union of parts, and 

 the epigynous mode of insertion. Irregularity also, since 

 it indicates specialization and adaptation to a particular 

 purpose, may be regarded as a mark of advanced evolution. 



302. Numerical Plan of Dicotyledons. In all the flowers 

 examined in Sections 295-300 except the first specimen, 

 the organs were found to be in fives, or multiples of five. 

 This is the prevailing number among dicotyledons, though 

 other orders are not uncommon, and occasionally even 

 trimerous forms like the magnolia, pawpaw, etc., are met 

 with. In the mustard family, in the common yellow 

 primroses of our old fields, and in several other well- 

 known species, the tetramerous, or fourfold arrangement 

 prevails, while some of the saxifrages, and a few other 

 plants are dimeroits, having their parts in twos. For the 



