1 62 Mr. Thomas Hick on 



hesitate to call the layer of narrow cells the endodermis, 

 and to regard the axis as monostelic. But at present this 

 has not been done. Nevertheless there are good reasons 

 for regarding this " line " as the boundary between the stele 

 and the cortex. In the first place, it is strongly suggested 

 by the typical species of Equisetum, e.g., E. arvense and 

 E. maximum, which, it will be allowed, are something more 

 than analogous. In the second it is supported by the mode 

 of origin and the development of the secondary xylem, as 

 will be shown later. 



Outside the " line " just dealt with, we have the cortical 

 tissues, which are here seen to present a considerable 

 amount of differentiation. At the first glance, indeed, it is 

 obvious that the cortex of this specimen has a remarkably 

 complex structure. It is made up of two layers or zones, 

 an outer and an inner, o, z, between which runs a dividing 

 line, which is undulated and roughly parallel to the surface 

 "of the stem. 



The inner zone is the broader of the two — having a 

 breadth of yJo inch — and is generally much better preserved. 

 In the middle of it the elements, though of different sizes, 

 are for the most part large and angular, and in shape and 

 arrangement are not unlike the xylem elements of the 

 vascular bundle of a fern. But the walls are not specially 

 thickened, and the cavities frequently contain black car- 

 bonaceous masses, m. Whether these represent special 

 substances, such as resin, tannin, or latex, or merely an 

 unusual accumulation of ordinary cell contents, it is 

 impossible to say. Between the black masses, which are 

 usually eccentric, and the distant cell wall, a faint concentric 

 line is often discernible, recalling the appearance of the 

 primordial utricle of recent plants. This, and the whole 

 appearance of the zone, seem to show that the contents of 

 these elements were introduced in the living state, and are 

 not mere infiltrations into empty cavities during the fossil- 



