TRANSLOCATION OF SUBSTANCES IN THE PLANT 341 



pressure flow exceeds that of diffusion 10 inillion times for a 

 distance of only 1 mm. 



From this physical scheme, let us pass to the .plant. Osmom- 

 eter A with the concentrated solution is represented by the leaf 

 cell, which under the influence of radiant energy continually 

 elaborates osmotically active substances, chiefly sugars. The 

 connecting tubes, along which the solution of sugar and of other 

 organic substance moves under the influence of pressure, arc 

 represented by the sieve tubes, the principal conductive elements 

 of the phloem. Osmometer B with the low osmotic pressure, 

 which squeezes out the water penetrating from the connecting 

 tube, is represented by the cells utilizing organic substances, in 

 which a low concentration is maintained either owing to growth 

 and respiration, as in the growing meristematic cells, or as a 

 result of the transformation of soluble substances into insoluble 

 ones, for instance, the transformation of sugar into starch, as in 

 cells of storage tissues of seeds and underground storage organs. 

 The reservoir that receives the excess of water is represented by 

 the dead water-conductive cells, the vessels of the wood, which 

 always accompany the phloem and serve not only for water 

 supply but likewise for the diversion of excessive water and thus 

 provide conditions for a continuous flow of the nutritive solution. 

 The tension usually prevailing in the vessels (see Art. 76) con- 

 tributes to a considerable degree to the exudation of the excess 

 of water from the developing cells. 



In the living plant, there exists a very essential distinction as 

 compared with the physical model examined above: viz., instead 

 of the direct communication between the two osmometers 

 through a glass tube, in the case of the plant there is a series of 

 cells separated from one another by septa and, what is still more 

 important, by impermeable layers of protoplasm. The sieve 

 tubes Hkewise consist of a series of cells separated from one 

 another by sieve septa. This distinction does not contradict but 

 only somewhat complicates the scheme presented above. In 

 spite of the septa, the sieve tubes possess one continuous cavity; 

 for the perforations in the sieve septa are so large that both the 

 protoplasm of the separate cells and the liquid cell sap represent 

 one continuous mass. Although the sieve septa increase some- 

 what the resistance to the flow of liquid, nevertheless they do 

 not present a serious obstacle. 



