600 PHANEROGAMS. 



influences their order of succession and their divergence from one another. But since, 

 notwithstanding the extraordinary variation of the other relations of form, the true 

 position of the floral leaves varies but little— though it may often be difficult to determine 

 — the knowledge of this position is often of great importance in the determination of the 

 affinities of the species, and hence for purposes of classification. This is especially 

 the case if we at the same time take into account the abortion of individual members 

 which is here of so common occurrence, the multiplication of the parts which take place 

 under certain circumstances, and their branching and cohesion. 



In order to fiicilitate a description of these relationships, it is necessary to explain 

 certain terms and methods of description. 



In the first place it is important to denote the position of all the parts of a flower 

 with respect to the mother-axis of the floral shoot. For this purpose the side of the 

 flower which faces the mother-axis is termed the posterior, that which is most remote 

 from it the anterior side. If a plane be imagined to divide the flower longitudinally from 

 front to back, and to include the primary axis of the flower as well as that of the 

 mother-shoot, this is the median plane of the flower, dividing it into a right and a left 

 half. Floral leaves, as well as ovules and placentae, which are bisected longitudinally 

 by the median plane, are said to have a median position, either posterior or anterior. 

 If another plane is now imagined at right angles to the first, and also including the axis 

 of the flower, it may be termed the lateral plane; this plane divides the flower into a 

 posterior and an anterior half, and parts which are longitudinally bisected by it are 

 precisely lateral. The two planes which bisect the right angle between the median 

 and the lateral planes may be called diagonal planes , and the parts which are bisected by 

 them be said to have a diagonal position. Flowers usually have some of their floral 

 organs placed exactly posteriorly or anteriorly, not so commonly exactly right and 

 left or exactly diagonally; but usually other additional terms must be used, such as 

 obliquely posterior or obliquely anterior. 



If next the position of the parts of the flower with respect to one another be ex- 

 amined, their arrangement, as has already been mentioned, is either spiral or -verticillate. 



Flowers with a spiral arrangement of their parts are comparatively rare, and appar- 

 ently occur only in certain orders of Dicotyledons (Ranunculaceae, Nymphaeacese, 

 Magnoliaceae, and Galycanthaceae). Braun has termed such flowers acyclic, when the 

 transition from one foliar series to another, as from calyx to corolla or from corolla 

 to stamens, does not coincide with a definite number of turns of the spiral (as 

 Nymphaeaceae and Helleborus odorus) ; hemicyclic when it does so coincide. This latter term 

 may also be employed when some of the foliar structures are actually cyclic (verticillate), 

 others spiral, as in Ranunculus, where the calyx and corolla form two alternating whorls, 

 followed by the stamens and carpels arranged spirally. Parts which have a spiral 

 arrangement sometimes occur in definite numbers, more often in larger indefinite 

 numbers. 



When on the other hand the parts of the flowers are arranged in whorls, the number 

 of the whorls, as well as that of the members of each whorl, is constant in the same 

 species, and within larger or smaller circles of affinity ^ When the number of members 



* [The number of whorls in a flower may vary very widely, from one {Carex) to fifteen or sixteen 

 (Aquilegia). In some cases the calyx, corolla, andioecium, and gynseceum each consists of a single 

 whorl, so that the flower has four whorls. More commonly, however, one or other of these series 

 consists of more than one whorl. This is most frequently the case in the androecium, so much so in 

 fact that it is customary to regard the typical flower as containing two whorls of stamens : in an 

 isomerous flower, if the stamens are in a single whorl it is said to be isostemonous, if in two whorls 

 diplostemonous, and so on. The calyx often consists of more than one whorl (Menispermaceae, 

 Berberidacese), and in most tetramerous flowers (Cruciferae, Onagraceae) it is composed of two 

 decussating dimoerous whorls. More rarely the corolla consists of more than one whorl ; instances 

 of this occur in the Fumariacese, Eerberidaceae, Papaveraceoe, Menispermacese. A gynseceum of two, 



