ii2 Mr. Thomas Hick on 



being pentagonal in some cases and hexagonal in others, is 

 only what we know to occur in recent roots. Moreover, in 

 well-preserved specimens it is difficult to resist the conclusion 

 that the vascular elements have developed centripetally from 

 the peripheral angles, as they are often arranged in radial 

 plates which converge towards the centre and increase in 

 size from without inwards.* In most cases these plates 

 actually meet in the centre, and more or less of metaxylem 

 is developed between them before secondary thickening 

 sets in, and in this way the mode of development is some- 

 what obscured. But less advanced stages are quite demon- 

 strative and readily enable us to make out the interpreta- 

 tion of the older. 



In the angles between the xylem plates, a group of thin- 

 walled elements is usually distinguishable, - ]- and though the 

 preservation of the fossils is scarcely so perfect as to show 

 distinctive phloem characters, there can be little doubt 

 that each group represents a phloem strand. 



The periphery of the stele is not as clearly marked ^s in 

 most recent roots, but in the place it should occupy there 

 are two, three, or a few layers of cells which are so distinct 

 from the cortical tissue outside them, that they may without 

 much danger of error, be regarded as representative of the 

 pericycle and endodermis.j 



The cortex itself, in its relative bulk, the absence of 

 marked thickening of the cell-walls — with the exception of 

 the elements with special contents — and the absence of 

 mechanical tissue in the hypodermal region, is also far more 

 suggestive of a root than a stem.§ In addition to this the 

 two peripheral layers, like those of the first type, accord 

 better with the piliferous and suberous layers of roots than 

 with the epidermal and sub-epidermal layers of stems.|| 



Finally Williamson's figure shows a rootlet arising from 



* PI. II., Fig. 2. a. f PI. II., Fig. 2. p. % PI. II., Fig. 2. d. 

 §P1. II., Fig. 2. b. ||P1. II., Fig. 2. c. 



