764 PHYSIOLOGY. 



under the influence of solar light. The absorption of oxygen 

 takes place in a still greater degree in the essential organs of 

 reproduction; hence, such an effect is more evident in perfect 

 flowers, than in those in which the stamens and carpels have 

 been more or less changed into petals — that is, when the flowers 

 have become partially or wholly double. It has been proved, 

 also, that staminate flowers absorb more oxygen than pistillate 

 ones. 



The combination which under the above circumstances takes 

 place between the carbon of the flower and the oxygen of the air, 

 is attended by an evolution of heat, which indeed is always the 

 case where active chemical combination is going on. This 

 evolution of heat in the majority of flowers is not observable, 

 because it is immediately carried off by the surrounding air ; 

 but in those cases, where many flowers are crowded together, and 

 more especially when they are surrounded by a leaiy structure, 

 such as a spathe, which confines the evolved heat, it may be 

 readily noticed. The flowers of the male cone of Cycas circi- 

 nalis, those of the Victoria regia, of several Cacti, and of many 

 Aracecs, present ns with the most marked illustrations. 



That this evolution of heat is dependent upon the combination 

 of the oxygen of the air with the carbon of the flower was con- 

 clusively proved by the experiments of Vrolik and De Vriese ; 

 for they showed that the evolution of heat by the spadix of an 

 Arum was much greater when it was placed in oxygen gas than in 

 ordinary atmospheric air, and that when introduced into car- 

 bonic acid or nitrogen gases it ceased altogether. 



Development of the Floral Envelopes. — The manner in which 

 the floral envelopes are developed may be shortly summed up as 

 follows : — 



They are subject to the same laws of development as the 

 usual foliage leaves, and make their first appearance as little 

 cellular processes, which grow by additions to their bases or 

 points of attachment to the axis. 



The calyx is always developed before the corolla. 



When a calyx is polysepalous, or a corolla polypetalous, the 

 component sepals or petals make their first appearance in the 

 form of little distinct papillae or tumours, the number of which 

 correspond to the parts of the future calyx or corolla. 



When a calyx is monosepalous, or a corolla monopetalous, 

 the first appearance of these organs is in the form of a little 

 ring, which ultimately becomes the tube of the calyx or corolla, 

 as the case may be. When these present lobes or teeth, as they 

 more commonly do, they arise as little projections on the top of 

 the ring, the number of which correspond to the future divisions 

 of the calyx or corolla. 



All irregular calyces or corollas are regular at their first 

 formation, the cellular papillse from which they arise being all 



