ROOTLETS. 25 



a ferruginous clay has occupied the vacant space and pressed the free, broken ends 

 of the bundles into close contact with the surface of the cylinder, instead of retaining 

 them in their normal positions. Beyond what we learn from fig. 37, none of my 

 specimens give me any further information respecting these bundles until we again 

 meet with them in Plate XII, fig. 41. This figure represents the inner surface of 

 a thin portion of the outermost bark of a Stigmaria, the opposite surface of which 

 displays the usual rootlet-scars. The free ends of several bundles have been left 

 exposed by the decay of the tissues through which they had passed, and intrusive 

 clay has acted upon them as upon those of fig. 39, though in the opposite direction. 

 I am indebted for these interesting fragments to Mr. B. Holgate, of Leeds, from 

 whom Professor Green obtained the specimen (Plate XII, figs. 38 and 39). 



We have now to examine the free rootlet as an external appendage to the bark. 

 These rootlets are long, cylindrical bodies, of nearly uniform diameter throughout 

 their length, which radiate with vegetative regularity from the entire circumference 

 of the root. Their length and diameter alike vary with their ages. The longest 

 examples which I have measured have been twelve inches in length, but other 

 observers record examples that have been fifteen. The greatest diameter attained 

 by any of my uncompressed specimens is -4 of an inch. 



Plate X, fig. 42, g', exhibits the usual appearance of their transverse sections. 

 It is one of six small rootlets which have found their way, through some accidental 

 opening, into the interior of the larger rootlet, g. The peripheral zone, g", consists 

 of parenchymatous cells, which in this instance have nearly uniform diameters, 

 though very frequently they increase in size from without inwards. Within this 

 cortical zone we have, with rare exceptions, the vacant space, g ; in the specimens 

 from our Lancashire and Yorkshire nodules this space is always occupied by the 

 white infiltrated mineral substance, which permeates all the vegetable fragments 

 found in the nodules, and which has contributed so materially to their exquisite 

 preservation. The probability is that these rootlets were fistular, as is the case 

 with the rootlets of the living Isoetes lacustris. I have seen no trace of tissue 

 occupying it even in the youngest and smallest rootlets met with, except at their 

 extreme bases where each rootlet is embedded in the exterior parenchyma of the 

 Stigmarian bark. Within the zone g" is the vascular bundle /', enclosed in a 

 cellular cylinder which technical accuracy can scarcely allow us to call a bundle 

 sheath, though it seems to act as one. A similar example of a transverse section 

 of a rootlet intruded within the medullary cavity of the Stigmarian root is seen 

 in Plate VII, fig. 14 (/' and g). 



But whilst the structure of sections like the two just referred to is very simple 

 and easily understood, a more complicated organisation exists in the basal parts 

 of these rootlets ; and longitudinal sections of these basal portions enable us to 

 understand much of their relations to the roots upon which they are planted. 



