CH. XIV] PLANES OF SYMMETRY, ETC. 123 



It may be noted that the flower, since it usually 

 opens out without elongating, and, unlike a bud, usually 

 spreads its parts, so that the observer can look down upon 

 it, lends itself far more readily to the plotting of a plan ; 

 consequently sections are rarely required, though they 

 also have their uses arid interest. 



If we regard from the front a lateral flower on a 

 vertical stalk, and agree to call the longitudinal planes 

 bisecting the flower as follows that passing vertically 

 through the flower and its stalk median, that at right 

 angles to this lateral, and any which is oblique to these 

 oblique we obtain three planes of symmetry of the flower 

 which can be made use of in descriptions in various ways. 



We have seen that the parts of an ordinary flower are 

 arranged round the floral axis, and some considerations of 

 importance occur in connection with the manner of this 

 arrangement. 



In a Lily, for example, we find three outer and three 

 inner similar segments of the perianth followed by three 

 outer and three inner stamens, and a gyncecium of 

 three carpels : that is to say, five whorls, in each of which 

 the three organs are similar in shape, size, texture, &c. 

 Moreover in each whorl or cycle the organs alternate with 

 those of the next whorl. Such a flower is cyclic (in this 

 case pentacyclic) with respect to the number of whorls, 

 and regular in regard to the symmetrically radial arrange- 

 ment of its parts : moreover, if we cut it vertically in 

 half with a sharp razor it will be found that it can be so 

 bisected in three planes, passing in each case through 

 the middle of a carpel, two stamens, and a sepal and 

 petal respectively, that each half is the mirror reflex of the 

 other. There are, in other words, three planes of symmetry 

 in this flower. In a Geranium, Primula (Fig. 29), &c., 

 there are five planes of symmetry in which the flower 



