78 CLASS-BOOK OF BOTANY. 
abrupt margins. The petals are contorted m all the hybridized 
garden species. | 
It is important to notice that, as a rule, petals are alternate 
with sepals, their margins being opposite the middles of the 
calyx-lobes, as is the case here. This feature, and similarly 
the position of the stamens with regard to the petals, can be 
accurately shown by the floral diagram, and must be carefully 
noted. The correct position of these floral whorls is most 
accurately fixed by drawing faint radii through the sepals, 
and subdividing the space between these by other radii, as 
shown by the dotted lines in the figure. 
It ig not advisable to attempt to show the adhesion of the 
whorls in the diagram, unless this can be done without sacri- 
ficing the clearness of the figure. Thus, when the petals are 
inserted on the calyx (perigynous) this may be represented by 
thin lines uniting the two like radii; but the adhesion of 
stamens to sepals, which is a common feature, cannot be shown 
by the same method. Again, epipetalous stamens can gene- 
rally be shown by a similar line, but no other feature of the 
adhesion should be attempted. It is, perhaps, questionable 
whether any great advantage is derivable from the construc- 
tion of such empirical diagrams, and whether the object 
sought to be attained would not be better secured by actual 
drawings of transverse and longitudinal sections. 
But the floral diagram is frequently used to indicate not 
only the parts of the flower which are present, but those also 
which are wanting—i.e., those parts which have been sup- 
pressed during development. Such theoretical diagrams can, 
of course, only be constructed when the development of the 
plant and of its allies is known, and they then become ex- 
tremely valuable as a concise method of showing and clearly 
explaining the mode in which the structure of the flower has 
been attained. In these diagrams the number and position 
only of the floral whorls are represented, without reference to 
their cohesion, adhesion, or xstivation; the position of those 
a 
Fig. 149. Floral diagram of Chickweed—empirical. 
