500 PART III. THE CLASSIFICATION OF PLANTS. 



lateral plane ; whilst the petals and the four inner stamens lie in 

 the diagonal planes. 



The number and the relations of the different parts of the flower 

 may be indicated not by diagrams only, but also by formalae in 

 which, as in the diagrams, for the sake of clearness, all the 

 peculiarities of form are overlooked. Thus the diagram Fig. 

 313 may be expressed by the formula Jt3, C3, J3 + 3, G^-\ which 

 means that the calyx K, and the corolla 0, each consist of a 

 single whorl of three members, the androecium of two whorls 

 each of three members, and the gynaeceum of one whorl of three' 

 members, all in regular alternation. When one whorl is super- 

 posed on another, the superposition is indicated in the formula by 

 a line | between the whorls. If the number of members in any 

 whorl is variable, the letter n is used instead of a number. Thus, 

 for instance, Ku, Cn, An + n, Gn is the theoretical formula of 

 most Monocotyledons. The absence of a whorl is expressed by a 

 cypher 0, and of individual members by the number of those 

 actually present. Thus the formula for the flower of a Grass 

 (Fig. 314) is KO, CO, ^3 + 0, (?\ Superior and inferior ovaries 

 are indicated by a stroke below or above the corresponding figure, 

 and duplication by the exponent 2; thus the diagram Fig. 315 

 is represented by the formula K2 + 2, C X 4, A2 -\- 2^, G^, 

 the X after C indicating that the position of the petals is diagonal, 

 i.e. that the four petals alternate with the four sepals, as if the 

 latter all belonged to the same whorl. The bracket in which the 

 number of the carpels of the gynaeceum G is enclosed, indicates 

 that the members thus bracketed are coherent. Staminodia may be 

 distinguished by a f before the figure. When the perianth is not 

 differentiated into calyx and corolla, it is expressed by the letter 

 P: thus the formula for the flower of Chenopodium is P5 | ^5 (r^. 



The Number of Members in a Whorl shows considerable varia- 

 tion : thus, in Monocotyledons it is generally three (rarely two or 

 five), whereas in Dicotyledons it is frequently five, less frequently 

 two or four, rarely three (e.y. most Lauraceae, Berberis, Rheum, 

 Polygonum). The number of members in a whorl is indicated 

 by the terms di- tri- tetra- penta-merous, etc. Whorls containing the 

 same number of members are said to be isomerous ; or, when the 

 number of members is not uniform, heteromerous. Flowers having 

 isomerous whorls are said to be eucyclic or isocyclic, whereas when 

 the whorls are heteromerous the flowers are said to be heterocyclic. 

 Of these two conditions the latter is the more common, though the 



