322 



PLANT PHYSIOLOGY 



milm 





The organic substances move along the sieve tubes in the 

 phloem cells of the conductive tissue of the bark. This can be 

 demonstrated by ringing experiments with woody plants; for 

 in these, there is a complete ring of xylem, transporting water 

 and surrounded by a complete ring of phloem, representing the 

 conductive part of the bark. It is interesting to note that plants 

 possessing sieve tubes both outside and inside of the xylem ring 



(as for instance the oleander) do not give clear-cut 

 results in ringing experiments, for translocation of 

 organic substances continues through this internal 

 phloem after the ringing. 



In some instances, the ascending stream may 

 carry organic substances. This occurs during the 

 spring, when the sap flows and the reserve sub- 

 stances stored in autumn, in the wood and bark of 

 the roots and stems, are digested rapidly and, pene- 

 trating in large quantities into the vessels of the 

 wood, are carried upward to the opening buds 

 under the influence of root pressure, which is very 

 intense at this time of the year. At other seasons, 

 the vessels of the wood contain very little 

 organic substance, which, as has been said, is 

 carried principally through the sieve tubes. 



The ascending transpiration stream moves 

 through the conducting vessels of the wood, which 

 branch showing are formed by vertical rows of dead cells that have 

 development of j^g^ their Contents. These form continuous tubes 



callus at the up- 

 per edge of the of Capillary dimensions by being joined through 



ringed region -^^ ^^ rcticulate Openings through their end walls. 



{after Brown). ^ x- o o 



Sieve tubes, through which the stream of organic 

 substances is translocated, are elongated living cells containing 

 protoplasm and provided with multiperforate transverse walls, 

 similar to a sieve. The mechanism of transport and forces that 

 move the streams are entirely different in xylem and in phloem, 

 and they must be discussed separately. 



74. General Nature of Water Translocation in the Plant. 

 Osmotic Movement of Water from One Cell to Another. — From 

 both a physiological and structural point of view, the course of 

 water movement in the plant may be divided into two parts. 

 Water moves mainly through the vascular system of the plant, 



Fig. 101.— a 



