80 * STUDY OF COMMON PLANTS. 



many species in which one or more of the whorls are 

 absent, and each is subject to more or less modification 

 of form and structure. 



Morphologically the flower is to be regarded as a modi- 

 fied branch, the members of its different whorls corre- 

 Its morphol- spending to so many leaves. The most obvious 

 sy- reasons for this view are that the flower has the 



position of a branch; that the arrangement of its parts 

 follows more or less strictly that of the leaves on the stem ; 

 that the anatomy of leaves and floral structures is essen- 

 tially the same ; that transitions from ordinary leaves to 

 floral envelopes are of frequent occurrence ; and finally 

 that reversions of parts of the flower to a more primitive or 

 leaf -like form often take place. 



It is convenient, and at the same time in accordance 

 with the view r s now held regarding the actual evolution of 

 Typical plant life, to take some such flower as that of 

 flower, the Trillium as a pattern or "typical" flower 



with which to compare others. The Trillium, as we have 

 seen, has three distinct green sepals, three petals, two 

 whorls of stamens of three each, and a pistil composed of 

 three parts, each part called a carpel. We may character- 

 ize our pattern flower, then, as having all the parts present, 

 these parts distinct from each other, of the same form and 

 size in each whorl, and presenting throughout the same 

 numerical plan, most frequently three or five. In other 

 words, it exhibits completeness, distinctness of parts, regu- 

 larity, and symmetry. 1 



The flowers of most plants differ in one or more respects 

 from such a typical flower as has been described. Never- 



1 The flower of Trillium departs slightly from the ideal typical flower 

 in the coalescence of the three carpels to form the compound ovary. Cf. 

 Gray, Structural Botany, pp. 176-178. 



