EPILOGUE 165 



cytoplasmic vesicles. The inhibitory effects of ultra-violet light and 

 ribonuclease on salt uptake can be attributed to destruction of RNA 

 v^'ith consequent failure of proteins to release their bound ions. 



Further information is needed about the precise mode of action 

 of chloramphenicol in preventing protein synthesis and salt absorp- 

 tion before the above suggestion can be accepted with confidence. 

 The validity of the overall hypothesis depends on the ability of 

 enzymes (or other proteins) from plant cells to combine with ions 

 with a specificity comparable to that exhibited by salt absorption, 

 and on the assumption that bound ions are released when proteins 

 complex with RNA. Experimental techniques through which these 

 possibilities may be examined, are being investigated. 



Consideration of the salt relations of intact vascular plants 

 introduces a further level of complexity arising from the anatomy of 

 tissues and organs. Some salt is apparently carried passively across 

 the root cortex into the stele with the transpiration stream via cell 

 walls. Active mechanisms are however also involved, and are often 

 of paramount importance, as is indicated by the high concentration 

 of salt frequently found in xylem sap, and by the selectivity of salt 

 transport from the medium into shoots. Active transport of salts 

 into the conducting elements of the xylem has much in common 

 with that of accumulation in vacuoles of parenchyma, and a greater 

 understanding of the latter will surely clarify our knowledge of the 

 former. The mechanism of movement of salts over relatively long 

 distances through the cytoplasm of unspecialized parenchyma is 

 not yet understood, but protoplasmic streaming may play some part 

 in transport of ions bound to protein, and small cytoplasmic vesicles 

 containing salt may move from cell to cell via the protoplasmic 

 connections. 



The primary distribution of salts in the stele takes place by mass 

 flow in the transpiration stream, and individual organs of the shoot 

 thus initially receive a supply of nutrients, in proportion to the 

 amount of water absorbed. Subsequent redistribution takes place 

 mainly in the phloem towards the regions of active growth. The 

 remarkable ability of growing cells to attract solutes seems to be 

 linked with a capacity to extract water and nutrients from the sieve 

 tubes, and may well depend on the intensity of protein synthesis or 

 turnover. 



Early enquirers embarked upon studies of salt absorption and 



12 M.S.A.P. 



