34 STIGMARIA FJCOIDES. 



There still remain for consideration some fragments of Stigniarige which, though 

 not capable of microscopic examination, could not have been interpreted without 

 the knowledge which we owe to the microscope. 



palaBo-phytology, I ask for some proof of the necessity for taking so extraordinary a backward step 

 and I am referred to some insignificant differences in the forms of the transverse sections of some 

 small, variable, vascular bundles. After other equally inaccurate statements respecling some morpho- 

 logical details, M. Eenault says : " Le deuxieme ordre de faisceaux offre sur un section transversale la 

 forme de triangle equilatere ou scalene. Fig. 2 et 6." " On distingue facilement sur deux ou trois 

 points, a, de la peripheric du faisceau, des tracheids de petit diametre, celles du centre etant beaucoup 

 plus larges, la section de ces dernieres est trois a quatre fois plus grande que celle des elements qui 

 composent l'ensemble des faisceaux du premier ordre decrit plus haut, et que nous regardons comme 

 appartenant a des organes foliaires ; de plus, les tracheides ne sont pas disposers en series rayonnantes 

 a partir de l'un des angles du triangle forme par la section, il est done evident que nous avous affaire a 

 une autre sort d'organe et que ces cordons vasculaires sont des faisceaux de racines " (loc. cit., 

 pp. 22, 23). 



In the above passages M. Eenault enumerates what he regards as three distinctive characters, by 

 means of which he recognises rootlet bundles. First, the unequal diameters of the Tracheids ; secondly, 

 the triangular form of the section of the bundle, and lastly, the absence of a radiating arrangement of 

 the vessels composing the bundle. It must be remembered that the bundles which he thus characterises 

 are, according to him, something distinct from those seen at fig. 14 at/,/', and consequently also distinct 

 from those seen in tangential sections like fig. 8, f, of Plate V. That these latter sections merely 

 represent two aspects of the same organ is beyond all question ; and since these are the only bundles 

 discoverable within, or arising from the vascular cylinder, we may ask, whence and where do these 

 apocryphal additional bundles arise ? Leaving this question, to which we get no answer, we may 

 inquire what value can be attached to the three other points ? 



1. Non-uniform size of the vessels. That this is a most variable feature I have already pointed 

 out in my previous descriptions. The same bundle varies even in different parts of its course. Thus, 

 in sections like fig. 9 nothing is more common than to find bundles, the Tracheids of which are derived 

 from the larger vertical tissues of the vascular cylinder, reduced to an extremely small size when 

 deflected, as at Plate VI, fig. 9,/ I affirm unhesitatingly that variations in the sizes of the vessels 

 composing a bundle cannot be depended upon as a differentiating character. 



2. Triangular form of the bundle section. Where the bundles emerge from the cortical surface of 

 the xylem cylinder and plunge into the bark, they almost always present more or less of the triangular 

 or wedge-shaped section ; but their arrangement becomes entirely changed as they leave the outer cortex 

 to enter the rootlets, where their transverse sections become more or less pyriform. But besides this 

 general fact, the forms of these sections vary considerably. We find many in which, as in Plate XI, 

 fig. 36, all the Tracheids are arranged radially; we have others, like Plate XI, fig. 61, in which only 

 the outermost ones thus radiate, and others again, like Plate XI, figs. 57 and 58, in which there is no 

 radial arrangement whatever. 



Definitions based upon such absolutely inconstant features are always worthless ; how much more 

 so when they are depended upon to distinguish organs so widely different, both morphologically and 

 physiologically, as roots and leaves ? They would be worthless, even did other facts suggest a probable 

 existence of a combination of x-ootlets and leaves on the axis of a Stigmaria ; but when, as is the case here, 

 all the known facts afford demonstrative evidence in the opposite direction, the employment of such 

 variable features, for the purpose of overthrowing the conclusions of two generations of experienced 

 palaeontologists, can scarcely be regarded as wise. 



In the first place, we have the most absolute vegetative uniformity in the orientation of all the 



