412 Boewig on the Histology and 



ciated with a slight amount of phloem. Internal to these 

 and widely separated from them by undifferentiated tissue 

 are two solitary wood cells, irregularly placed. All these 

 wood elements are decidedly thickened, although the primary 

 membrane is well marked. 



The protoxylem early splits up into three and much later 

 into a greater number — six to eight — of patches. The pro- 

 tophloem patches remain five in number so long as there are 

 three patches of protoxylem. 



As soon as the phloem begins to develop hard bast, the 

 true phloem disintegrates, and its place is taken by internal 

 phloem. Thus are formed five canals external to the wood. 

 These canals are more evident and considerably larger in the 

 side shoots and tendrils than in the main axis or larger stems. 

 In the larger stems there is also more hard bast than in the 

 side shoots and tendrils. This seems to show that the 

 reduction in amount of true phloem is not a mechanical 

 result of the formation of bast, but is rather in line with 

 the usual reduction of external phloem in twining stems. 

 The internal phloem develops all around the protoxylem. 



The wood spreads until its original patches are quite con- 

 fluent around the stem, leaving no trace finally of recog- 

 nizable medullary rays. By this time the external phloem, 

 now consisting of little more than hard bast, whose cell 

 lumen is also by this time almost completely obliterated, 

 together with a trace of flattened phloem cells, has broken 

 into perhaps twenty patches of varying size, large and small 

 ones frequently alternating. The smaller of these have no 

 canal and no soft bast. The internal phloem also consists 

 of six to ten pointed triangular patches, each with two or 

 three protoxylem cells enclosed. 



The development of the bordered pits is clearly traceable 

 through all stages and makes a very interesting study. 



In young side shoots, and to a less extent in tendrils, the 

 cortex remains more abundant and somewhat more columnar 

 than in older ones. 



