156 PLANT PHYSIOLOGY 



chlorophyll-granule is temporarily deposited in the cell, the 

 conversion of these into portable compounds is advan- 

 tageous. Although oil will pass through cell-wall and 

 living protoplasm, the same amount of nutritious material 

 will pass through the same distance much more rapidly 

 as sugar dissolved in water. Starch, being insoluble, 

 is not only innutritious as such, but is not portable, and 

 hence it must be converted into a soluble substance, also 

 sugar. What is true of the non-nitrogenous foods elabo- 

 rated in those organs receiving the special form of energy 

 needed for their manufacture, is also true of the nitrogenous 

 foods, elaborated probably in all the living cells of the 

 plant, whether illuminated or not. The nitrogenous foods, 

 if temporarily deposited in insoluble form in the cells elabo- 

 rating them, will be much more portable if converted into 

 soluble forms. As we have already seen, the non-nitro- 

 genous foods are transferred mainly as sugars, the nitro- 

 genous mainly as amides. 



We have seen (p. 116) that the transfer of water and 

 dissolved mineral salts from the absorbing organs to those 

 parts in which they are used as food or from which water 

 is evaporated, would be too slow to secure an adequate 

 supply if the movement were wholly osmotic. In order to 

 satisfy the needs of cells consuming food, and to free the 

 manufacturing cells from elaborated food so that they may 

 continue to make it, the transfer of foods must also be by 

 more rapid means than by osmosis merely. In small and 

 simple plants the manufacture, consumption, and storage of 

 food may go on in the same cells simultaneously. In larger 

 plants, division of labor and differentiation of tissues secure 

 greater efficiency and economy. In these plants there dif- 

 ferentiates a system for distributing elaborated foods from 

 the places of manufacture. The earliest and simplest con- 

 ducting system is for this purpose. We find such a system 

 well developed in the large marine algae (Fucus, Laminarix, 

 Nereocystis, etc. ) . When plants come upon the land it be- 

 comes hard to obtain water and easy to lose it. When plants, 

 by becoming erect, limit their water-absorbing part to one 

 end and place their food-manufacturing cells at the oppo- 



