128 GENERAL BOTANY 



the ducts and are combined by the leaf with the carbon dioxide 

 from the air to form starch, sugar, and nitrogenous foods. As 

 long as the plant is young and growing this leaf-made food will 

 be used for immediate growth, but as the leafage increases, an 

 excess of food will be formed daily over and above that used 

 for the immediate needs of the organism. This excess of food, 

 stored in the leaves during the periocf of active photosynthesis 

 by day, is digested by the leaf cells at night in the manner 

 already indicated for the cotyledons during seed germination. 

 The soluble sugar and protein thus formed then moves down the 

 phloem portion of the veins and of the vascular bundles of 

 the stem of the bean plant by the process of osmosis, which 

 is the physical method of movement of all soluble foods in 

 plants. As they move downward in the phloem of the main 

 vascular bundles they are absorbed along the way by the cortex 

 cells and also pass horizontally along the wood rays toward 

 the pith, where, as we have seen, food is often stored. In an 

 annual plant, like the bean, little food is permanently stored in 

 the stem since it is used mainly for seasonal growth and for the 

 production of seeds. As soon as the bean flowers begin to 

 develop they form centers of great activity in growth, especially 

 during the formation of the pollen and the young seeds, or 

 ovules. The food stream then begins to be diverted to the 

 flowers, in consequence of the growth activities going on in the 

 developing anthers and ovules. As soon as fertilization has 

 taken place the young seeds begin to form endosperm, and this 

 process necessitates a constant supply of soluble sugar and pro- 

 tein. Since osmosis takes place from points of greater concen- 

 tration to those of less concentration for any given substance, 

 the cells of the growing cotyledons of the bean must needs 

 convert the sugar and soluble protein into insoluble starch 

 and protein grains in order to reduce the concentration of solu- 

 ble foods in their water vacuoles ; otherwise the flow of food 

 toward these cells would soon cease. This conversion of soluble 

 sugar into insoluble starch is done by leucoplastids in the cells of 

 the cotyledons, while storage protein granules are formed in the 

 general cytoplasm of the cells. Fats and oils also seem to be 



