230 



FUNCTIONS OF FLORAL ENVELOPES. 



taining a moderate or definite number of ovules. Thus, in the ovary 

 of Leguminous plants (fig. 429), the ovules o, are attached to the 

 extended marginal placenta, one above the other, forming usually two 



parallel rows corresponding to each 

 margin of the carpel. When the 

 ovules are definite (uniform and 

 can be counted), it is usual to find 

 their attachment so constant as to 

 afford good characters for natural 

 orders. When the ovules are very 

 numerous or indefinite, while at 

 the same time the placenta is not 

 much developed, their position ex- 

 hibits great variety, some being 

 directed upwards, others downwards, 



others transversely (fig. 430), 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 Cryptogamous plants, in place of ovules there are cellu- 

 lar bodies called spores, to which allusion will be made when the seed 

 is considered. 



4. FUNCTIONS OF THE FIX5RAL ENVELOPES. 



473. 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 carbonized substance caUed chlorophylle. They 

 are consequently concerned in the assimilation of matters fitted for the 

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

 The corolla, along with the thalamus and disk, is concerned rather 

 with development than with respiration. Hence it does not in general 

 produce chlorophylle, nor does it give off oxygen. It protects the 

 essential organs, and eliminates carbonic acid by a process of oxidation. 

 The starch granules contained in it, as well as in the thalamus and 

 disk, are not altered by the respiratory process, so as to become more 

 highly carbonized, but are oxidized, so as to be converted into saccha- 

 rine matter. The quantity of oxygen absorbed was determined by 

 Saussure. He found that double flowers absorbed less in proportion 

 to their volume than single flowers; that the essential organs contained 

 more oxygen than the floral envelopes; and that the greatest absorp- 

 tion took place when the stamens and pistil were mature. 



Fig. 429. Carpel or legume of Ononis rotundifolia, with several campylotropous ovules, o, 

 placed one above the other. /, Funiculi. a, Base of the style. 



Fip. 430. - Loculament of the ovary of Peganum Harmala, with numerous ovules, o, attached 

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



