240 NUTRITIVE METABOLISM 



centers of activity in the chloroplasts, or by independent leuco- 

 plasts. During the night the diastase found in the plastids 

 hydrolyzes the starch, converting it into glucose or maltose, 

 which then may pass out of the cell to be again condensed a 

 number of times in its way to the root, or deposited in a storage 

 tissue by the action of other plastids (leucoplasts). Other carbo- 

 hydrates, such as reserve cellulose, may be acted upon similarly. 

 The proteids are subjected to the action of peptonizing ferments 

 which convert them into soluble form and enable them to diffuse 

 as described above. 



307. Channels for the Conduction of Plastic Material. The 

 transportation of material from the point at which it is absorbed, 

 or formed, to the tissues in which it is used, or stored, is accom- 

 plished in a variety of ways. The mineral constituents taken up 

 by the roots pass through the root hairs and cortex into the 

 xylem of the roots, and then pass upward through dead cells dif- 

 fusing laterally by osmotic attraction to the embryonic and cortical 

 tissues. The movement of carbohydrates and other complex 

 bodies takes place most rapidly through the sieve cells, and other 

 elongated elements in the phloem, also diffusing laterally into the 

 cortex. In trees the lateral movement is made largely through 

 the medullary rays. Elongated cells with a comparatively small 

 number of septae facilitate the process. In the lower plants with 

 undifferentiated tissues, conduction must be accomplished by 

 osmosis through cells of small diameter, and is sufficient here to 

 move material through the distances intervening between the dif- 

 ferent parts of the body. The streaming movements of proto- 

 plasm doubtless aid in the process, although such movements are 

 not sufficiently prevalent to be considered as a general factor in 

 the process. Involuntary movements of the liquids in cells due 

 to bending and twisting from the force of the wind, or water, must 

 also be of benefit in translocation. Latex and resin are present 

 in quantity in many plants and are capable of transportation 

 through systems of tubes and canals continuous without partition 

 walls throughout the body of the plant. 



