278 ME. B. G. CORMACK ON POLTSTELIG 



Fig. 5 represents a transverse section cut at a distance of 150 miliim. from the apex. 

 The incomplete endodermis appearing as small arcs indicates that the structure is that 

 of type four. 



Fisr. 4 shows a stasre in the transition as seen in a section cut 115 miliim. from the 

 apex. 



Fig. 3 illustrates that at 77 miliim. from the apex the structure is simpler and 

 approximates to that of type three, which has a single endodermis complete and wavy 

 in its outline. 



Fig. 2 was drawn from a section cut 15 miliim. from the apex, where the structure is 

 another stage in the transition, being practically that of type two. 



The root from which fig. 6 was drawn had not attained sufficient length to illustrate 

 fully the transition between complex and simple structure. However, between two 

 transverse sections from parts not far separate considerable difference in degree of 

 complexity could be seen. With a wider field for search a single root might well be 

 found exhibiting at successive points examples of each of the five structural types which 

 have just been distinguished for convenience. 



This raises the question as to the nature of the histological changes associated with 

 morphological differences in successive parts of an individual root. Two explanations 

 offer themselves for examination : — 



1st. Secondary changes may have produced the complex abnormality. 



2nd. The apical meristem may have undergone continuous change in its mode of 

 differentiation. According to the first view an abnormal part was once normal: 

 according to the second it never was. 



The fact that greater complexity is found in the basal older parts, less complexity in 

 younger apical regions, with continuity of transition, is not at variance with either view. 



The histological evidence obtained in the examination of these views is illustrated in 

 part by PI. XIX. figs. 7, 8, 9, 10, 11, 12. 



Figs. 7, 8, 9 are diagrams showing the disposition of tissues in the neighbourhood 

 of the points indicated by the arrows in the diagrams 3, 4, 5 respectively. Xylem and 

 sclerenchyma are shaded dark ; phloem and parenchyma, light. Larger vessels of the 

 wood are shown unshaded and with double contour lines; large intercellular spaces, 

 unshaded and with dotted contours. Endodermis is represented by a dark line. 



Figs. 10, 11, 12 are drawings illustrating in greater detail tissues round the points 

 indicated by the arrows in figs. 3 and 7, 4 and 8, 5 and 9 respectively. 



If, according to the first view, increased complexity results from secondary changes, 

 modifying dispositions which were simpler when younger, careful observation should 

 detect histological evidence of such change ; but the only peculiarities noticeable are 

 such as might be expected from the development of sclerenchymatous masses in the 

 neighbourhood of parenchyma. 



Against this view, then, there is the negative evidence that, hypothetically, changes of 

 great complexity, seemingly without parallel in any shoot, have been accomplished 

 without leaving any trace of the process. 



Further, there is evidence of a more positive character. The transverse section repre- 



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