ii INTRODUCTION. 



the corresponding cylinder in the aerial stems. The centrifugal cylinder of the 

 latter must obviously have been continuous with the similar one of the root, but the 

 centripetal zone enclosed within the centrifugal zone obviously terminated at the 

 base of the stem, like the medullary sheath of an ordinary exogen. But how it does 

 so yet awaits demonstration. Connected with this another problem arises. Judging 

 from the specimens in our possession I should conclude that the diameter of the 

 vascular cylinder of a Stigmaria was about one fourth that of its entire diameter, 

 including its surrounding bark. The largest vascular cylinder I have yet seen is 

 recorded in my text as being y§ of an inch in diameter, which would give to the 

 root of which it was the centre, a diameter of about 4-f inches, or a circumference 

 of rather more than 14 inches. But in the text I have described roots, the proxi- 

 mal ends of which are more than three feet in circumference. Now, such roots, 

 according to the scale of proportions adopted above, would require a vascular 

 cylinder fully three inches in diameter. We have not yet found any that approach 

 this magnitude. We have yet to discover whether or not any roots of this size 

 exist, the internal structure of which has been preserved. Unfortunately little 

 more than the outermost bark remains either in stems or roots of these dimensions. 

 Then we want specimens showing the structure of the part of the cortex internal to 

 the prosenchymatous layer, which latter forms so conspicuous a feature alike in the 

 Stigmarian roots and in their aerial stems. We are familiar with the parenchy- 

 matous zone that occupies this position in the stems, and we have indications that 

 the same parenchyma was continued into the root ; but we want clearer evidence 

 than we yet possess whether or not such was the case. 



It must further be borne in mind that all the numerous Carboniferous plants, 

 great and small, had rootlets of some kind, and that some of them bear a superficial 

 resemblance to those of Stigmaria. This is especially the case with the rootlets 

 of the plant which I named Amyelon, yet none of these can well be mistaken for 

 those of Stigmarise, though I doubt not that some have been so mistaken. 



Some readers may consider that I have dwelt needlessly upon minute details 

 of structure which, in their opinion, have neither interest nor importance. This is 

 possible ; yet I have done so with a definite hope as well as with an object. The 

 hope is that sooner or later we shall know more than we yet do of the morphological 

 and physiological links connecting the primaeval forms of vegetation with their living- 

 representatives. No real facts, however small, can be absolutely worthless to the 

 botanist who strives to work out this subject. Anyhow, the faithful record of 

 them can do no harm, and they may be found useful at some future time. 



Two peculiarities may be observed in the form of this Monograph. One is the 

 elaboration of the Index to the Plates. In studying the works of some of our 

 most distinguished Botanists, I have found it difficult to discover in what part of the 

 text the detailed descriptions of the Plates were to be found, and the too brief refer- 



