292 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
development. The general relations of the tissues composing these rootlets to their 
larger exogenous roots were shown in my memoir, Part II. , Plate 31, fig. 52. I now only 
call attention to their relationship to the rootlets of the living Selaginellce. Fig. 14 
represents the transverse section of one of these Stigmarian rootlets in the condition 
in which they most commonly occur. At a is a vascular bundle composed of barred 
vessels enclosed eccentrically within a small cylinder composed of the cells of the 
innermost cortex c. At d we have only the outer cortical layer, the intermediate 
tissues being rarely preserved; but these missing elements have resisted decay in 
fig. 15, in which example we have the space between the outer cortex d and the inner 
one c, occupied by a very delicate cellular tissue e, the three layers seen in this 
specimen apparently corresponding with the three cortical layers b, d, and e, of the 
branch, fig. 2. The line of demarcation between the outer and middle layers is 
very sharply defined. Fig. 16 represents a very small rootlet in an early stage 
of growth ; its entire diameter not being more than '025. Its vascular bundle, a, only 
consists of four very small vessels attached eccentrically to one point of the inner side 
of the ring of inner cortex, c, whilst all that remains of the middle and outer bark is a 
small band of the former, e, and a very thin ring of the latter, d. The narrow band of 
middle bark, e, connecting the cylinder of inner-bark cells with the outer cortical zone 
is of common occurrence in this form. In the specimen figured, some of the cells of 
the outer zone have become partially detached from the outer ring. It will be 
observed that the diameter of the entire young rootlet is less than that of the central 
inner bark cylinder of a matured rootlet represented in fig. 18. 
I fortunately possess proof that these small rootlets belonged to very young roots. 
The enormous thickness to which the exogenous Stigmarian roots were capable of 
attaining is well known, and since these thick roots often appear to terminate abruptly 
like the end of a cucumber, it has been thought by some that they never were of small 
dimensions. This, however, is an error. The youngest plants can have had no true 
Stigmarian exogenous root but only rootlets, and the stage of growth at which the 
exogenous structures first made their appearance has yet to be discovered. It however 
appears certain that the latter were primarily of very small size, and furnished with 
proportionally minute rootlets, each of which had but a correspondingly small number of 
vessels. I have before me a section of the bark of a very small Stigmarian root, from 
Mr. Aitken’s cabinet, the entire diameter of which does not exceed ‘2, and which is 
giving off several rootlets whose average diameter is about '04. No vascular bundle 
in the rootlets of this specimen possesses more than from three to five very small 
vessels. The same cabinet contains a beautiful little section of the exogenous vascular 
O 
axis of a Stigmarici, whose maximum diameter is '6. This has, of course, been the 
centre of a larger root than that just described. It gives off numerous root-bundles 
proceeding towards the periphery of the bark, each of which possesses from ten to 
fourteen vessels. These two specimens are the smallest examples of Stigmaria that I 
have yet met with. On the other hand, in two large Stigmarian exogenous cylinders 
