TRANSLOCATION OF CARBOHYDRATES 263 



the evidence available, resting as it does mainly upon anatomical 

 data, cannot be regarded as more than indicative of what may be 

 rather than of what are the actual paths of translocation of 

 assimilates. 



The Phloem 



Sieve-tubes. — The phloem is characterised by the presence of 

 sieve-tubes, which, as their name implies, are formed from 

 elongated cells whose end walls are perforated in the mature 

 state. Their development, in the case of certain Dicotyledons, 

 has been studied by A. W. Hill in great detail. He found that 

 in the young state the end walls of the cells are pierced by 

 numerous protoplasmic threads arranged in groups in thin 

 portions or pits (fig. 3). As the cell develops the wall thickens 

 except in the pits and changes occur in the connecting threads, 

 probably brought about, as Gardiner has suggested, by the 

 action of an enzyme. This appears to travel from both ends of 

 the thread towards the middle lamella of the wall. In some way 

 or other this ferment changes the nature of the central core of 

 the protoplasmic thread and the wall through which the thread 

 passes, but leaves a tube of unaltered protoplasm (figs. 4, 5). 

 The central strand which is enclosed in the protoplasmic sheath 

 is known as a " slime-string," while the substance into which the 

 cellulose wall has been changed is called " callus." By the 

 continued action of the enzyme the slime-strings are thickened, 

 until finally the whole of the membrane at each of the thin 

 portions of the transverse wall becomes converted into callus 

 and is disorganised, each original group of fine slime-strings being 

 replaced by a single thicker one, which with its sheath of 

 protoplasm entirely fills the resulting callus-lined perforation in 

 the end wall (figs. 6, 7). On the completion of the process only 

 the thickened portions of the wall remain, forming a sieve of 

 cellulose, which during the active period of the sieve-tube is 

 thinly coated with callus. Hence the protoplasmic lining and 

 the slimy contents of the mature cells are connected through 

 these pores and a tube with transverse sieves results, extending 

 continuously through the plant (fig. 8). 



When young each element contains a large nucleus, which 

 generally disappears later on, though not in all cases according 

 to some investigators. Embedded in the parietal protoplasm 

 denser granules called " leucoplasts " are frequently to be found. 



