102 MORPHOLOGY, OR COMPARATIVE ANATOMY. 



arrangement (spiral, opposite, or verticillate, &c.), freedom or union, form 

 (regular or irregular), order of growth (consecutive, simultaneous, inter- 

 calary, definite, indefinite or interrupted, congenital or postcongenital, &c.). 

 These changes may be congenital and hereditary, and then common to all 

 plants that have originated from a common ancestral type j or acquired 

 or adaptive, when they have become manifest in order to fulfil certain 

 special or individual requirements, or to put the plant in harmony with 

 the circumstances under which it has to live. Thus the form, colour, 

 and perfume of flowers are often in direct relation to the habits and 

 structure of the insects which visit them for the sake of the honey, and 

 whilst so engaged effect the fertilization of the flower in ways hereafter 

 to be mentioned. It may thus be said that the form of the plant and its 

 parts is dependent, 1st, on hereditary endowment, and 2nd, on adaptation 

 to the work it has to do, the means it has of doing it, and the circumstances 

 under which it must be accomplished. Sometimes from causes only im- 

 perfectly understood there is a reversion from a more complex or adult to a 

 simpler or embryonic form, as when a petal or a stamen becomes leafy ; 

 and other cases of similar character may sometimes be explained hypo- 

 thetically by assuming them to be reversions to an ancestral form. 



Diagrams, Floral formulae. For purposes of ready comparison, 

 and to avoid lengthy descriptions, diagrams or plans and floral for- 

 rnulse are made use of. 



A diagram is intended to show the number, arrangement, and relative po- 

 sition of the parts of the flower. Thus, fig. 162, p. 91, represents the diagram 

 of a complete, regular, isomerous, pentamerous flower. Fig. 161, p. 90, shows 

 a trimerous flower, with the parts in regular alternation. Diagrams of this 

 kind are spoken of as empiric when tney represent the actually existing 

 state of the flower, while they are termed theoretical when the condition 

 shown is that assumed or known to be the typical one, apart from the modi- 

 fications brought about by abortion, chorisis, &c. Thus, fig. 165, p. 95, 

 shows the usual condition in Labiates, where there are four stamens, the 

 situation of the fifth, which is abortive, being shown by the dotted circle. 



In place of diagrams floral for mulce are sometimes made use of. These 

 are constructed in various ways according to the views of various authors, 

 though it would be convenient if uniformity of practice could prevail in 

 this matter. The following illustrations will exemplify these formula3 j 

 thus a regular pentamerous encyclic flower may be represented thus : 



S5 P5 A5 G5; 



the S representing the calyx of five sepals, P the corolla of five petals, 

 A the andrcecium of five stamens, and G the gynsecium or pistil of five 

 carpels, each whorl distinct from each other, and the parts of each indi- 

 vidual whorl also distinct and free from cohesions or adhesions so-called. 

 In the instance given, the parts are assumed to be all in their proper 

 alternate position ; but this might be more clearly shown thus : 



So A5 

 P5 G5 



or more briefly thus : 



