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412 I FOURTH GROUP.— SEED-PLANTS. 



branching in the inflorescence are generally different from those in the vegetative 

 stem, so the positions of the leaves in the floral shoot of the Angiosperms are not 

 generally the same as those on the vegetative parts of the same plant. The cessation 

 of the apical growth of the torus and its considerable expansion or even hollowing out 

 before and during the formation of the leaves of the perianth and the sporophylls have 

 an inevitable effect on the order of their succession and on their divergences. But while 

 all other relations of form vary to an extraordinary degree, the true position of the 

 foliar structures of the flower, though difficult to determine, is liable to a comparatively 

 small amount of variation ; and a knowledge of this position is often of great use in 

 ascertaining affinities and therefore in classification, especially if we take into account 

 at the same time the frequent abortion of the separate parts, their multiplication 

 under certain circumstances, and their branching and cohesion. 



To render the representation of relations of this kind more easy, we must have 

 recourse to certain diagrams and symbols. 



It is important first of all to show the position of all the parts of the flower with 

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

 which is turned towards the mother-axis is called the posterior^ that which is turned 

 away from it the anterior side ; if we now imagine a plane (longitudinal section) 

 passing through the flower from front to back and including the axis of the flower and 

 the axis of its mother-shoot, this is the median plane or section of the flower, and it 

 divides it into a right and a left half. The foliar structures of the flower and the ovules 

 and placentas, which are halved longitudinally by the median plane, are said to 

 be in a median position, either median posterior or median anterior. Again, if we 

 imagine a plane at right angles to the median plane and also including the axis of 

 the flower, it may be termed the lateral plane ; it divides the flower into an anterior 

 and a posterior half, and the parts of the flower which are bisected longitudinally by 

 it are exactly right and left. Two planes which bisect the right angle between the 

 median and the lateral planes may be called diagonal planes, and parts of the flower 

 bisected by them are said to be diagonally placed. Foliar structures in flowers exactly 

 posterior or exactly anterior are not uncommon ; they are much less frequently placed 

 exactly laterally or diagonally, and we must generally have recourse to other expressions 

 as obliquely posterior or obliquely anterior. 



As regards the relative positions of the parts of the flower, it has been already stated 

 that their arrangement is either spiral or in whorls {cyclic). 



Flowers with their parts arranged spirally appear to be comparatively rare and to 

 be confined to certain orders of Dicotyledons (Ranunculaceae, Nymphaeaceae, Mag- 

 noliaceae, Calycanthaceae). We may follow Braun in calling them acyclic when the 

 passage from one foliar structure to another, as from calyx to corolla and from corolla 

 to androecium, does not coincide with definite turns of the spiral, as in Nymphaeaceae 

 and Helleborus odorus ; if it does so coincide, Braun names them hejnicyclic, a term 

 which may also be used when some of the foliar structures of a flower are cyclical, and 

 others spiral, as in Ranunculus., in which the calyx and corolla form two alternating 

 whorls and are succeeded by the spirally arranged sporophylls. Parts of the flower 

 with a spiral arrangement are sometimes present in small and definite numbers, usually 

 in larger and indefinite numbers. 



If on the other hand the parts of the flower are arranged in whorls, the number of the 

 whorls and the number of members in each whorl is usually a definite one in the same 



or three leaves each or a multiple thereof. To this numerical relationship the term sym7netry has 

 been applied by English and French botanists, a syf?imet}-ical Jlower being one exhibiting this 

 numerical relationship, while in an unsymmetrical flower the relationship fails in one or other 

 of the whorls. In estimating this symmetry the gynaeceum is not taken into account. This 

 specific use of the expression symmetry must be borne in mind in view of the wider and more general 

 use of the term in biology and of its application to the flower by Sachs on page 423 of this book.] 



