GENERAL CONSIDERATIONS. Ill 



gressively metamorphosed leaves, called respectively the calyx, 

 the corolla, the stamens, and pistils. Of these four verticils, 

 the two outer whorls marked a and b, in Fig. 16, are called 



floral envelopes, and are considered to be merely accessary 

 organs, whose functions are to protect the two inner whorls, 

 the stamens and pistils marked c and d, which are named 

 sexual organs, and which are by far the most important and 

 highly organized parts of the flower. A flower may be perfect 

 and reproduce itself without either calyx or corolla, but not 

 without stamens or pistils; for these last organs are imme- 

 diately connected with the formation of the seed, the germ of 

 the future plant, and without these secreting and all-important 

 bodies, it is impossible for fertilization to take place, or seed to 

 be produced. 



The leaves of the flower, like those of the stem, are arranged 

 spirally about the axis of growth, and therefore the separate 

 pieces of each verticil alternate with each other. Thus the 

 petals or leaves of the corolla alternate with the sepals or leaves 

 of the calyx ; that is to say, each petal is placed in the inter- 

 val between two sepals ; the stamens alternate with the petals 

 and the pistils with the stamens. 



The sepals of the calyx, or outermost of the floral whorls, 



