584 
DR JOHN M‘LEAN THOMPSON ON 
continuation of parenchymatous tissue from the immediate neighbourhood of the 
slender persistent medulla into the mass of parenchymatous xylem. Within this 
expanded pith there then appeared an irregular and incomplete endodermal tube 
and an isolated chain of endodermal cells (I.E.). The resulting structure is illus- 
trated in fig. 7. This condition led on to that shown in fig. 8 ; the inner endodermal 
tube had become closed so as to isolate a body of parenchyma from the main 
medullary mass. The independent chain of endodermal cells was still present in 
the sections. Further forward the inner endodermal tube became open once more, 
and new endodermal formations arose within the medulla. The resulting structure 
appeared in section as in fig. 9. By the opening of the inner endodermal tube the 
parenchyma which it surrounded at the point immediately preceding was again 
placed in open communication with the main medullary mass. An independent row 
of endodermal cells lay within the partially enclosed parenchyma, and a sheet and 
slender tube of endodermal nature had arisen towards the opposite side of the pith. 
A number of fluctuations in the disposition of the inner endodermal formations 
followed, but these soon gave place to the arrangement shown in fig. 10. A wide 
cylinder of inner endodermis was now clearly defined, but locally it was imperfectly 
differentiated, while isolated endodermal cells appeared sporadically in various 
situations. One such cell is shown within the parenchyma enclosed by the inner 
endodermal tube. As the series was followed still further forward the sporadic 
endodermal formation ceased. The wide and continuous inner endodermal tube was 
maintained for some distance, though still locally imperfectly differentiated (fig. ll). 
But a return to an ill-defined condition was soon made, the continuous cylindrical 
inner endodermis giving place to an interrupted tube within which detached chains 
of endodermal cells were enclosed. The resulting structure appeared in section as 
in fig. 12. The inner endodermal formation had been fundamentally disrupted. 
These transformations prefigured the condition shown in fig. 13. The inner endo- 
dermal formation had been resolved as the sections were followed forward into a 
narrow and almost complete tube enclosing a large body of the medulla, and into 
an independent irregular group of cells. This latter group was found to diminish 
and finally to end abruptly in the sections immediately succeeding the one figured 
(fig. 13). The condition depicted in fig. 14 was subsequently established, the 
persisting inner endodermal tube having been completely formed though narrowed 
considerably. Around the inner endodermis a general development of parenchy- 
matous inner xylem had been organised. It will be apparent that at this point 
the medulla was again diminishing, and the inner endodermis was in fact once more 
in process of being eliminated. The chief steps in this elimination as shown in the 
sections are represented in figs. 15, 16. The inner endodermis became a slender 
tube within a very restricted mass of pith (fig. 15), and when the vanishing point 
for the endodermis was passed, a stelar structure in no way distinguishable from a 
medullated protostele was again established (fig. 16). The purely parenchymatous 
