226 FLOWERS AND THEIR WORK. 



257. Symmetry of the Flower. Nearly all flowers are 

 symmetrical, i.e. can be divided into two corresponding 

 halves. When it can be so divided along one plane only it 

 is said to be bilaterally symmetrical or zygomorphic, but 

 when the flower may be equally divided in several planes 

 it is radially symmetrical or actinomorphic. In regarding 

 flowers from the biological point of view, we may restrict 

 these terms to the perianth. A flower that can be visited 

 from any side has a radially symmetrical (" regular- ") 

 mechanism, whereas that of a flower which can be 

 properly visited from one side only is zygomorphic ("ir- 

 regular.") 



258. The Floral Diagram. The general structure of a 

 flower and the arrangement of its parts may be graphically 

 represented by means of drawings of vertical and horizontal 

 sections. The latter are always drawn as a ground-plan 

 showing all the parts in the same figure, and the term 

 " floral diagram " is applied to them. 



It is always of great importance in such diagrams to cor- 

 rectly represent the position of the flower with regard to the 

 stem and bract connected with it. The face of the flower 

 turned towards the stem is called the posterior face, that 

 towards the bract is the anterior one. Cohesion of parts 

 may be indicated by connecting lines. Difficulty is often 

 experienced in correctly orienting the flower with regard to 

 the stem, and it is useful to remember that there is a single 

 posterior sepal in most Dicotyledons, if we except the 

 Leguminosae and also a few plants in which the posterior 

 sepal has not developed. The parts of the typical Monocoty- 

 ledonous flower are arranged in regular cycles of three, and 

 the odd sepal is usually anterior. 



Make the diagram of good size (at least 2 ins. in diameter) . 

 To get a transverse section of a small ovary, do not remove 

 the sepals, etc., but cut right across the flower, holding the 

 flower upside down, and cutting thin slices with a razor or 

 sharp knife (beginning at the base of the flower) until the 

 interior of the ovary is exposed. In cutting a flower longitu- 

 dinally through the middle, it is usually best to begin the cut 

 at the base of the flower. 



