ABSORPTION AND MOVEMENT OF WATER 157 



site end of the body, they must supply themselves with a tis- 

 sue system for the rapid conduction of water and salts from 

 roots to leaves. The wood, which carries mainly food mate- 

 rials, is so much more conspicuous than the bast, which car- 

 ries foods, that the wood seems the more important and the 

 earlier needed of the two. This may be true of land-plants, 

 but it is not true of aquatics. Phylogenetically the food- 

 conducting system is the older; in aquatics it is the main 

 or only tissue for the rapid transfer of aqueous solutions. 



The anatomical distinctions between wood and bast 

 are so evident that it is easy to infer that there is per- 

 fect division of labor between these two sets of tissues. 

 But the living cells of the bast need mineral salts, perhaps 

 to maintain turgor (p. 99), to neutralize injurious by- 

 products (p. 100), to assist in the construction of proto- 

 plasm (p. 101) — and wiU receive, transfer and use them 

 just as the living cells of the wood need sugar and amides 

 as food and will receive, transfer, and consume them. 



From illuminated chlorophyll-containing cells the carbo- 

 hydrates not laid down in solid form like starch or in slowly 

 transferable form like oil, will pass by exosmosis to adjacent 

 cells containing less. Such osmotic transfer will continue 

 while the chloroplastids are manufacturing diffusible food 

 and until osmotic equilibrium has been attained. Osmotic 

 transfer will begin so soon as the carbohydrate deposited as 

 starch or oil is converted into diffusible form, and it will con- 

 tinue so long as starch and oU are converted into sugar. 

 From cell to cell in the leaf parenchyma, and from this into 

 the cortical parenchyma of branch and stem, osmotic transfer 

 will take place ; but it will take place always most rapidly in 

 the direction of least osmotic pressure. This will obviously be 

 toward and into the sieve-tubes of the vascular bundles. The 

 sieve-tubes — composed of comparatively large, long cells with 

 thin lateral walls and perforated cross-walls — are continu- 

 ous for considerable distances, often forming, by means of 

 anastamoses, a system uninterrupted from tip to base of the 

 plant. In the sieve-tubes, as in the ducts and tracheids, 

 there will tend to be less pressure than in the adjacent cells. 

 In the ducts and tracheids the danger of collapse is avoided 



