BRACTS AND FLORAL LEAVES. 85 



Thus the members of contiguous whorls {e.g., sepals and petals) 

 alternate with one another, while the members of alternate whorls 

 are superposed, i.e., are in the same rank or orthostichy. There 

 are ten ranks in the example supposed. Comparatively few 

 flowers are constructed so regularly as this, but a great many 

 can be explained by supposing alterations to have taken place 

 either in the number of whorls or the number of parts in certain 

 whorls. Very commonly both kinds of change appear to have 

 happened. Abundant examples will be found in the sequel. 



We have now to consider in detail calyx, corolla, andrcecium, 

 and gyncecium. Five headings may be conveniently taken in 

 each case, viz., Number and Arrangement, Cohesion (union of 

 like parts), Adhesion (union of unlike parts), External Charac- 

 ters, and Structure. 



The Calyx, or outer whorl of the perianth, departs less from 

 the foliage leaves in nature than any other part of the flower, 

 and a gradation may sometimes be traced between sepals and 

 such leaves. In the dog-rose, for example, there are five sepals 

 arranged in a very short spiral. The two or three lowest bear 

 small leaflets in a pinnate manner, thus resembling the pinnate 

 foliage leaves of the same plant. 



Number and Arrangement.— In acyclic flowers, such as those of 

 Cacti, the spirally arranged sepals may be indefinite in number, 

 and pass, on the one hand, into bractlets, on the other, into 

 petals. The cyclic and hemicyclic flowers of dicotyledons are 

 generally characterized by the numbers five and four (or multiples 

 of the same). The former number very frequently indeed goes 

 with a phyllotaxis of two-fifths. Examine once more the wild 

 rose. The foliage leaves are here arranged with the above 

 divergence (rf. p. 50), which means that each cycle includes 

 five leaves and turns twice round the stem. The five-leaved 

 calyx is here just such a cycle telescoped, but the spiral arrange- 

 ment can still be made out. A little further change would alto- 

 gether obliterate the spiral, and give us a whorl of five members, 

 as in buttercup. The number four is found in wallflower, stock, 

 shepherd's purse, and many other plants. An examination of 

 the calyx in a wallflower will show that the sepals are in two 

 alternating whorls, each containing two sepals. This may per- 

 haps correspond to the opposite decussate arrangement found in 

 foliage leaves (p. 49). In wallflower, however, the foliage leaves 

 are not arranged in this manner, which fact tells against the ex- 

 planation. Three (or a multiple of it) is by far the commonest 

 number among monocotyledons (fig. 37). The three large white 

 sepals of snowdrop furnish a striking example. This number is 

 associated, as might be expected, with a phyllotaxis of one-third. 



