CHAPTER VIII. STRUCTURE OF THE FLOWER. 65 



same number prevails throughout, as, for instance, the number 

 four in the order Cruciferae, and the number three, or some 

 multiple of it, in the great group of Monocotyledons. A flower 

 constructed on the plan of one. or which possesses one sepal, 

 one petal, etc., as is sometimes the case, is called monomerous ; 

 one whose parts are in twos, dimerous ; one whose parts are in 

 threes, trimerous ; one whose parts are in fours, tetramerous ; 

 one whose parts are in fives, pentamerous ; and one whose parts 

 are in sixes, hexamerous. The commonest of these arrangements 

 are the trimerous, the tetramerous, and the pentamerous. It 

 must be observed that in some cases, owing either to the multi- 

 plication, suppression or coalescence of the parts of some whorls, 

 the numerical plan is more or less obscured, but it may in most 

 instances be discovered by careful study. 



Such a flower as is represented in Figs. 202 and 203, is tri- 

 merous ; since, also, it possesses all the parts which properly 

 belong to a flower, it is complete ; because it has the same 

 number of parts in each whorl, and these whorls alternate with 

 each other regularly, it is symmetrical ; because the parts of each 

 whorl are similar in size and shape, it is regular ; because it 

 possesses all the parts essential to the production of seed, namely 

 stamens and pistils, it is hermaphrodite, or perfect ; and because 

 it possesses both sets of the floral envelopes, calyx and corolla, 

 it is dichlamydeous. The flower of the Trillium, except that it 

 has its stamens in two whorls of three each and its three pistils 

 partially united, illustrates very well a typical trimerous flower, 

 and the flowers of the Flax and Stonecrop are pentamerous 

 flowers that closely conform to the typical flower in structure. 

 But, while we have reason to believe that most flowers are modi- 

 fications of some such form as we have described as typical, the 

 flowers that deviate from this form are far more numerous than 

 those which conform to it. Most of these deviations we are to 

 regard as adaptations or modifications which the organs have 

 undergone in relation to the surroundings of the plant, or the 

 conditions of its existence. Even such very irregular and un- 

 symmetrical flowers as those of the Orchids were doubtless 

 derived from perfectly regular and symmetrical ones by slow 

 processes of change reaching back through many thousands of 

 generations. Evidence that this is the fact is obtained from com- 



