LESSON 13.] THE RELATIVE POSITION OF ITS PARTS. 



93 



&c., also have their flowers constructed on the plan of four as to the 



calyx and corolla, but this number is interfered with in the stamens, 



either by the leaving out of two sta- 

 mens (which would complete two sets), 



or in some other way. Next to five, 



the most common number in flowers 



is three. On this number the flowers 



of Lily, Crocus, Iris, Spiderwort, and 



Trillium (Fig. 189) are constructed. 



In the Lily and Crocus the leaves of 



the flower at first view appear to be 



six in one set ; but the bud or just- 



opening blossom plainly shows these to consist of an outer and an 



inner circle, each of three parts, namely, of calyx and corolla, both of 



the same bright color and delicate texture. In the Spiderwort and 



Trillium (Fig. 189) the three outer 



leaves, or sepals, are green, and dif- 

 ferent in texture from the three inner, 



or the petals ; the stamens are six 



(namely, two sets of three each), and 



the pistils three, though partly grown 



together into one mass. 



247. Alternation of Parts, The symmetry of the flower is likewise 



shown in the arrangement or relative position of successive parts. 



The rule is, that the parts of successive circles alternate with one 



another. That is, the petals stand over the intervals between the 

 sepals ; the stamens, when of the same number, 

 stand over the intervals between the petals ; or 

 when twice as many, as in the Trillium, the 

 outer set alternates with the petals, and the 

 inner set, alternating with the other, of course 

 stands before the petals ; and the pistils alter- 

 nate with these. This is shown in Fig. 189, 



and in the diagram, or cross-section of the same in the bud, Fig. 190. 



And Fig. 191 is a similar diagram or ground-plan (in the form of a 



FIG. 189. Flower of Trillium erectum, or Birthroot, spread out a little, and viewed from 

 Above. 



FTG. 190. Diagram or ground-plan of the same, as it would appear in a cross-section of 

 the bud ; the parts all in the same relative position. 



FIG. 191. Diagram, or ground-plan, of the Flax-flower, Fig. 174. 



