258 



FUNCTIONS OF FLORAL ENVELOPES. 



ment so constant as to afford good characters for classification. When 

 the ovules are very numerous or indefinite, while at the same time the 

 placenta is not much developed, their position exhibits great variation, 

 some being directed upwards, others downwards, others transversely 



(fig. 466), and their form is altered by pressure into various polyhedral 

 shapes. In such cases it frequently happens that some of the ovules 

 are arrested in their development and become abortive. In Crypto- 

 gamous plants, in place of ovules there are cellular bodies called spores, 

 to which allusion will be made when the seed is considered. 



4. — Functions of the Floral Envelopes. 



The bracts and calyx, when of a green colour, perform the same 

 functions as leaves, giving off oxygen under the influence of light, and 

 producing the substance called chlorophyll or phytochlor. They are 

 consequently concerned in the assimilation of matters fitted for the 

 nutrition of the fiower, and they aid in protecting the central organs. 

 The corolla does not in general produce chlorophyll, nor does it give 

 off oxygen. On the contrary, it absorbs oxygen from the air. At 

 the same time there is a conversion of starch into grape sugar, an 

 evolution of carbonic acid gas, and in many instances a very marked 

 elevation of temperature, caused by the combination between the 

 carbon of the flower and the oxygen of the air. The starch, which is 

 stored up in the receptacle and at the base of the petals, by passing 

 into the state of dextrin and grape sugar, becomes fitted for vegetable 

 nutrition. Important purposes are thus served in the economy of the 

 plant. The saccharine or honey-like matter which often collects in 



Fig. 463. Carpel of Nuttallia eerasoides, with two suspended collateral ovules, o, One of 

 the ovules. /, Funiculus, s. The base of the style. Fig. 464. One of the loculaments of 

 the ovary of .fflsculus hybrida, laid open to show two ovules, o o, inserted at the same height, 

 but turned in different directions, m m, Micropyle indicating their apex, s. Base of the 

 style. Pig. 465. Carpel or legume of Ononis rotundifolia, with several campylotropous 

 ovules, 0, placed one above the other. /, Funiculi, s, Base of the style. Fig. 466. Locu- 

 lament of the ovary of Peganum Harmala, with numerous ovules, d, attached to a projecting 

 placenta, p, and pointing in different directions, s. Base of style. 



