22 VEGETABLE PHYSIOLOGY. 



The flower is the forerunner of the fruit, the fruit is the medium 

 in the heart of which the seed is developed. The org^ans which 

 constitute the flower are the calyx and the corolla, destined to sup- 

 port, nourish, and protect the pistil and the stamina, which are the 

 essential parts ; the calyx is a green membrane which surrounds the 

 corolla, and in certain flowers replaces it. 



7 he corolla is monopetalous or polypetalous according as it is com- 

 posed of one or of several pieces The stamens occupy the interioi 

 of the corolla ; they are terminated by summits of a vascular tex- 

 ture ; these are the anthers ; the powder which covers and sticks 

 slightly to them is designated under the name of pollen. 



The pistil placed in tlie middle of the flower is composed of the 

 ovary, the style, and the stigma. 



The ovary encloses the germ, the embryo of the seed ; but this 

 embryo is only developed by the action of the pollen. The style is 

 in some sort the tubular prolongation of the ovary ; it supports the 

 stigma, which is the glandular part that receives the fecundating 

 influence of the pollen. 



From what has now been said, the pistil may be considered as the 

 female orjjan of the flower, the stamens as the male organs. 



Many flowers combine the organs of the two sexes. These flow- 

 ers are hermaphrodites ; those which oidy contain one organ, are 

 called unisexual. Both male and female flowers are produced to- 

 gether on certain plants ; in others, the flowers are all only of one 

 sex, male or female. Polygamous plants are those which show a 

 union of male and female flowers, or which have hermaphrodite 

 flowers on the same steuj. 



In some flowers, the sexual organs at the period of fecundation 

 acquire the property of motion, so as to facilitate this grand act 

 The stamens, for example, are seen in certain plants to approach 

 the stigma, to deposite tlieir pollen on it, and then to withdraw. It 

 occasionally happens again that stamens, which are at first naturally 

 in a position inclined with reference to the pi&lil, become suddenly 

 straightened in such a way as to cast their pollen on the female or- 

 gan, after which they resume their original position. In certain 

 flowers a very considerable evolution of caloric has been perceived 

 on the approach of the period of fecundation. In some arums, for 

 example, the temperature has been observed to rise to 40" and even 

 50° cent. (104" to 12-2'' Fahr.) It is probable that this phenomenon 

 is quite general, and that it only varies in point of inleni>itv- 



Fecundation accomplished, the office of the flower is at an end. 

 It colIaj)ses, withers, and dies. But the impregnated ovary enlarges 

 by degrees, until it has attained maturity, when it presents two dis- 

 tinct parts, which by their union compose the fruit : these parts are 

 the pericarp, and the seed — the husk or shell and the gram. The 

 pericarp always surrounds the seed ; but it sometimes happens that 

 It is so thin and delicate that it blends with the seed. 



The germination of seeds, the evolution of new plants, is only 

 accomplished under certain physical conditions which demand our 

 consideration. 



