POLLEN I ZATION, CROSS-FERTILIZATION, ETC. 145 



457. Ripening of Fruits. After the fruit has attained its full growth the process 

 of ripening commences, during which the pulp becomes gradually sweetened and softened, 

 chiefly by the change of the starch into more or less of soluble sugar. Thus ripening is 

 to the pericarp what germination is to the seed. In its earliest stage the pericarp con- 

 sists of structure similar to that of green leaves, composed of cellular, vascular, and 

 woody tissues, and epidermis with stomata. Its distended growth afterward results from 

 the accumulation of the flowing sap, which here finds an axis incapable of extension. 

 Thus arrested in its progress it gorges the pistil and adjacent parts, is condensed by ex- 

 halation, assimilated by their green tissues, which still perform the office of leaves. Con- 

 formation goes on rapidly within, and the excess of cellulose is deposited in the cells'aa 

 starch. Oxygen is usually absorbed in excess, acidifying the juices. 



458. In the same way we account for the production of honey 

 in the flower. Copious deposits of starch are provided in the 

 receptacle and disk ( 85). At the opening of the flower, this 

 is changed to sugar, to aid in the rapid development of those 

 delicate organs which have no chlorophyl wherewith to assim- 

 ilate their own food. The excess of sugar flows over in the form 

 of honey. The wise economy of the honey is seen in fertiliza- 

 tion. For, attracted by it, the insect enters the flower, rudely 

 brushes the pollen from the now open anthers, and inevitably 

 lodges some of its thousand grains upon the stigma ! 



459. Experiment has proved that in all these cases of the formation of sugar from starch 

 oxygen is absorbed and carbonic acid evolved a process which we might expect, since 

 starch (Ci a HI O O 10 ) contains proportionably more carbon than sugar (C, 9 H, a O 13 ) con- 

 tains. It is probable that these two phenomena in vegetation are always co-existent. 



Review. 447. May a plant consist of a single cell ? An example. Two modes of devel- 

 opment. 448. How is growth distinguished from reproduction f 449. What is the embry- 

 onic vesicle ? Its origin ? Its destination ? Whence this new impulse ? 450. Trace the 

 growth of the pollen-grain. Trace the course of its tube. 451. Different views of this. 

 Show by the figures. 453. Development after fertilization. 453. Course of the pollen in 

 the Conifers. 454. State of the embryo in the seed. Provision for its nourishment. 

 455. What chemical changes ensue ? Whence the yeast? The sugar? The heat? 4T>fi 

 What is formed in ripening fruits ? 458. Whence is honey ? Its economy ? 



CHAPTER VIII. 



POLLENIZATION, CROSS-FERTILIZATION, ETC. 



45 9a. In 111 (also in 449) it is stated that pollen is essen- 

 tial to the fertilization of the flower. We may here add that in 

 order to fertility, pollen must not only be produced, but must 

 also in some way be conveyed to the stigma, and lodged on its 

 surface. Another requisite is that the pollen and pistil shall 



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