STIGMARIA 251 



the calcareous nodules. Their structure is on the 

 whole very uniform, though their dimensions vary 

 greatly, the diameter ranging from a centimetre to a 

 millimetre or less. The external cortex, which is 

 generally well preserved, is several cells in thickness, 

 and is often divided into an outer and inner zone by a 

 band of thick-walled tissue. Sometimes the inner zone 

 as a whole has thicker walls than the outer (Fig. 105), 

 but often there is an entire absence of differentiation 

 (Fig. 101). These distinctions may possibly turn out 

 to be of specific value, but at present we have no clue 

 to their significance. As we have already seen, all the 

 layers of the cortex in the rootlet are continuous with 

 the corresponding tissues of the main Stigmaria, and 

 present similar characters. The outermost layer of the 

 rootlet is sometimes papillose, but no true root-hairs 

 have been observed. 



Indications of tangential division are sometimes 

 found in the cells of the external cortex, which may 

 thus have undergone some slight secondary increase in 

 thickness. Within this external zone we almost always 

 find a wide empty space, from which all tissue has 

 perished. It is rare, except at the base of the rootlet, 

 to find any remains of the middle cortex, which must 

 once have bridged the gap. Occasionally, however, 

 especially in young rootlets, some thin-walled tissue is 

 still preserved in this region, or a single, stout trabecula 

 may connect the inner with the outer cortex (Fig. 103). 



The internal cortex forms a ring, of very small size 

 compared with the outer layer, and immediately en- 

 closes the vascular strand (Figs. 1 o 1 - 1 o 5 ). As, after the 

 decay of the intermediate tissue, there was nothing to 



