1920] ' CRIBBS—TODEA BARBARA 281 



in this instance is expressed by the fraction ? . It later changes to 

 a f arrangement, which appeared most frequently in the early stem. 



A solid protostele occurs a short distance above the fifth trace, 

 and is illustrative of a constant recurrence of a perfectly solid stele 

 associated with the departure of either the fifth or sixth foliar 

 strand (fig. 18). The appearance of the protostele at the detach- 

 ment of the fifth trace is shown in fig. 28, which is typical of this 

 level of the stem in all of the young plants studied. It will be noted 

 that the xylem is composed of tracheae alone, although internal 

 parenchyma was present both above and below the three preceding 

 nodes, and is constantly present above the level of the sixth trace. 

 Although the occurrence of internodal parenchyma is common 

 in the young stele, the number of elements is usually very limited. 

 Occasionally, however, the internodal pocket becomes quite ex- 

 tensive (fig. 1), when the central cylinder assumes the appearance 

 of a true siphonostele with centrally placed thin-walled elements. 

 Yet despite the siphonostelic aspect, the stem becomes distinctly 

 protostelic at the node above, and also at the level of exit of the 

 fifth and sixth traces. The parenchyma pocket shown in fig. 16 

 is quite the normal condition, and is seen to be a basipetal extension 

 from the cleavage of the stele at the node above. 



It has already been pointed out that the departure of a trace 

 from the protostele may so influence the xylem that it becomes 

 entirely divided into two segments. A recurrence of this is shown 

 in fig. 19, in which instance, however, the indentions of both the 

 fifth and sixth traces are concerned, affecting the stele from opposite 

 sides. The two residual strands coalesce immediately above the 

 node as in the former instance cited (fig. 12). The eighth trace 

 takes its departure from the stele in a similar manner to the fifth, 

 leaving a solitary intruded parenchyma cell which is soon replaced 

 above by tracheae, leaving a solid protostele (fig. 20). Even at 

 nodal regions the stele was always observed to contain thin-walled 

 elements above the level of the eighth leaf, while more commonly 

 the fifth or sixth node marks the upper limit of a stele entirely 

 free from these elements. 



There is a gradual increase in the number of xylem elements 

 accompanying the enlargement of the stem up to the time of 



