92 MOEPHOLOGT, OR COMPAKATIYE ANATOMY. 



may be merely coalescence of the margins of organs of the same 

 whorls (cohesion), or confluence of normally distinct whorls (ad- 

 hesion). These so-called unions are generally the consequence 

 of arrest of development, owing to which, parts usually separate 

 in the adult condition remain inseparate. 3. Unequal growth or 

 degree of adhesion in the organs of particular whorls, producing 

 irregularity. 4. Irregular growth either of the receptacle, or pro- 

 duction of outgrowths from various organs by enation. 5. (Sub- 

 stitution of one organ by another (metamorphosis). 6. Superposi- 

 tion, where parts usually alternate are placed opposite, or, more 

 correctly, are superposed the one to the other. 



Dr. A. Gray has furnished an interesting illustration of these laws of 

 modification, from a family (Crassulaceae) in which different kinds of 

 deviation occur together with examples of very symmetrical flowers. In 

 Crassula (fig. 162) is found a symmetrical pentamerous flower, with five 

 sepals, five petals, five stamens, and five pistils, all regularly alternating, 

 and only slightly confluent at the base. In Tillcca some species have four, 

 some only three organs in each whorl, but the flowers are still regular and 

 symmetrical. In iSedum (Stonecrops, &c.) the flowers of some species 

 are pentamerous, those of others tetramerous ; but here the number of sta- 

 mens is doubled by the introduction of an entirely new circle of these organs 

 (multiplication). Rochea has the margins of its petals slightly coherent, 

 while in Grammanthes the petals and sepals are respectively coherent 

 more than halfway up. Cotyledon has coherent envelopes, and a double 

 series (multiplication) of stamens as in Sedum, to which is added an ad- 

 herence of the stamens to the tube of the corolla. In Penthorum the five 

 styles are coherent firmly together below, while in some cases its petals 

 are suppressed. In Sempervivum (Houseleek) the number of sepals, petals, 

 and pistils varies in different species from six to twenty, and the stamens 

 from twelve to forty. 



Pleiotaxy, or multiplication of the number of whorls, is very 

 common, especially as regards the stamens. In the trimerous 

 flowers of Liliacea3 and AmaryllidacesB there are six stamens stand- 

 ing in two circles of three. In the Poppy family the tetramerous 

 circles are still more multiplied ; and in the Kose, Buttercup, &c. 

 we have further examples. When the number exceeds three or four 

 circles of one kind of organ, the organs are said to be indefinite in 

 number, and the verticillate arrangement becomes very indistinct 

 in the opened flower. In the White Water-lily (Nymphcea) we 

 have multiplication both of petaline and staminal circles ; and in 

 Magnolia, Ranunculus, &c. the pistils are much multiplied, exhibit- 

 ing in these a distinctly spiral arrangement. 



Multiplication of circles occurs abnormally in the double flowers of 

 gardens, in which we often find far more organs than exist in the normal 

 state, as in Daffodils and other flowers where the organs are naturally few 



