OF THE ORGANS OF REPEODrCTION. 763 



which have the whole petiole early exposed to the air grow 

 much more towards the upper part of the petiole." 



It will be seen that the above results of M. Trecul differ in 

 several important particulars from those of De Mercklin, and 

 that the development of leaves is by no means such a simple 

 and uniform process as was supposed by him. Further investi- 

 gations are, however, still required before we can be said to have 

 arrived at altogether certain conclusions upon the subject. 



Section 3. — Physiology of the Organs of Repeoductiox. 



Having now briefly alluded to the special functions of the 

 elementary structures, and of the organs of nutrition, we pro- 

 ceed, in the next place, to treat of the special functions of tlie 

 organs of reproduction ; but those who may desire to complete the 

 account of the nutritive fimctions may pass at once to Chapter 

 2 (p. 790), which treats of the General Physiology of the Plant. 



1. Functions of Bracts and Floeax Envelopes. — One of 

 the principal offices performed by these organs is, to protect the 

 young and tender parts placed within them from injury. When 

 green, as is usually the case with the bracts and sepals, their colour 

 is due to the presence of chlorophyll in their component cells, and 

 they then perform the same functions as ordinary leaves. When 

 coloured, however, as is generally the case with the petals, and 

 occasionally with the bracts and sepals, they appear to have, in 

 conjunction with the thalamus, a special function to perform ; 

 which consists in the formation of a saccharine substance from 

 the amylaceous matter stored up in them. This saccharine 

 matter is designed more especially for the nourishment of the 

 essential organs of reproduction. That such is the function of 

 these parts seems to be proved by the varj-ing composition of 

 the thalamus at different periods of the flowering stage. Thus, 

 at the period of the opening of the flower, the thalamus is dry 

 and its cells are filled with amylaceous matters; as flowering 

 proceeds, these matters become converted into saccharine sub- 

 stances, upon which the surrounding parts are nourished ; and, 

 finally, after' flowering, it dries up. In fact, a similar change 

 takes place in the process of flowering to that which occurs in 

 germination, where the amylaceous matters are in like manner 

 converted into saccharine. 



When the saccharine matter is in excess, during the process of 

 flowering, it is found upon the parts in a liquid state, and may 

 be removed vrithout the flower suffering. 



In this conversion of amylaceous into saccharine matter, 

 oxygen is absorbed from the atmosphere, and carbonic acid gas 

 given off in a corresponding degree. Hence, the action of the 

 coloured parts of the flower upon the surrounding air is directly 

 the reverse of that of the leaves and other green organs whilst 



