THE FLOWER 11 



can be arranged illustrating the gradual development and dif- 

 ferentiation of the two floral envelopes. Foliar members, 

 whether derived from foliage leaves or sporophylls, become 

 more and more definitely associated with the sporophylls, until 

 they may be regarded as constituting an inconspicuous, bract- 

 like perianth. They gradually appear in two definite sets and 

 become more conspicuous, and sooner or later show the petaloid 

 texture and coloration. The final stage is a completely differ- 

 entiated calyx and corolla, with their characteristic differences. 

 This tendency to produce a completely differentiated calyx and 

 corolla has resulted in its attainment by most flowers, but there 

 are numerous cases in which even near relatives have not made 

 the same progress in this regard. For example, the phenom- 

 enon styled apetaly may be observed in flowers whose nearest 

 relatives have a distinct calyx and corolla. While some cases 

 of apetaly may be explained as the suppression of a set of floral 

 envelopes, there are certainly cases in which it means that the 

 two sets have never become differentiated. This indicates that 

 progress made in a single direction can not be used as a cri- 

 terion of relationship. In general, however, it must remain 

 true that a flower with completely differentiated calyx and 

 corolla, other things being equal, is of higher rank than a 

 flower which has not attained this differentiation. 



Among the most primitive flowers the floral axis tends to 

 elongate, and the members appear in indefinite numbers along 

 a low spiral. In more highly developed flowers the growth of 

 the axis in length is checked at a very early period, so that the 

 spiral along which the members successively appear becomes 

 lower and lower, until it has only a theoretical existence, pass- 

 ing into successive cycles, which eventually become limited in 

 number. With the appearance of definite cycles the number of 

 members appearing in each one becomes limited, the limit in 

 Monocotyledons being prevailingly three, and in Dicotyledons 

 five or four. It is to be noted that the cyclic arrangement is 

 not attained simultaneously by all the sets of a single flower. 

 For example, in many species of Ranunculus the sepals and 

 petals are cyclic, or approximately so, while the stamens and 

 carpels are distinctly spiral. This tendency is so well-marked 

 and so uniformly displayed that Engler has used it as a basis 

 for dividing Monocotyledons into two great series, the " spiral 



