PHYSIOLOGICAL PROCESSES DURING FLOWERING 445 



between the separate elements change substantially. Tlu^ 

 potassium content decreases, while that of magnesium and 

 especially of phosphorus increases. This largin accumulation of 

 phosphorus in ripening seeds explains the great influence of 

 phosphate fertilizers upon the yield of grain. Nitrogenous 

 fertilizers, as has been noted previously, increase the yield of 

 the total dry weight of the plant. Phosphorus penetrates into 

 the seeds in the form of inorganic phosphates but is deposited 

 in the form of organic phosphorus compounds, chiefly lipoids and 

 phosphatides. 



The ripening of fleshy fruits is accompanied like the ripening 

 of seeds, by an accumulation of nutritive substances, but the 

 latter serve not for the nutrition of the embryo, but for the 

 attraction of animals, which disseminate the seeds. Substances 

 with a palatable taste predominate in fruit, chiefly sugars, 

 pectin compounds, starch, organic acids, and flavoring esters. 

 Usually, the ripening of the seeds coincides with the ripening 

 of the pericarp. So long as the seeds are not mature, the peri- 

 carp is acid or astringent in flavor owing to the presence of a large 

 amount of organic acids and tannins. With ripening, these 

 substances are replaced by sugars. At the same time, essential 

 changes occur in the physical properties of the fruit. Previously 

 hard, they now become soft, this being a result of the change in 

 nature of the pectins, which cement together the cells of the flesh. 

 Under the influence of enzymes, they become soluble in the water 

 impregnating the cell walls; the cells lose their firm intercellular 

 cement, and their shape changes from polyhedrons to become 

 more or less spherical. Such a softened tissue can be more 

 readily eaten by animals and more easily digested by the ali- 

 mentary juices. Other changes in structure and chemical 

 composition of the cell walls and cell contents increase the 

 palatabiUty of fruits. 



In the first stages of development of apples and other succulent 

 fruits, the sugars translocated from the leaves are deposited in 

 the cells of the flesh of the fruits in the form of starch. This 

 contributes to a slight extent to their hardness and to the absence 

 of a sweet taste. In later stages of ripening, this starch is trans- 

 formed into sugar, the amount of which increases rapidly in the 

 fruit. In the so-called ''winter varieties," these transformations 

 take place only after the removal of the fruit from the tree, 



