Anatomy of Acrostichum aureum. 179 
bility of secondary cortex arising, for the production of special air¬ 
absorbing tissue. 
Turning now to fossil roots, we find that they had very fre¬ 
quently characters which we regard as appertaining more strictly 
to shoots. The large Calamite roots have always a good pith and 
extensive cortex (see above). The anatomy of Stigmarian axes is 
of so indeterminate a type, that some divergence of opinion has 
arisen as to their morphological nature. It is best perhaps frankly 
to own them intermediate in character, like the rhizophores of their 
living allies, the Selaginellas. 
The prevalence of centripetal development of the xylem in the 
shoots of Vascular Cryptogams removes one of the most constant 
differentiating features between the root and shoot as known in 
the Higher Plants. Nevertheless it seems to be only in the 
Lycopsida that other characters combine to render morphological 
limitations obscure. 
The peculiar nature of the periderm of fossil plants is fre¬ 
quently remarked upon. In root and shoot alike it arises from the 
cells of the outer cortex; while “‘ periderm’ and ‘ bark ’.were 
very different in nature and function from the recent tissues which 
answer to them morphologically.” It is doubtful whether any of the 
periderm cells became suberized, for the tissues exterior to it 
remained fresh, and it is conceivable that some of its delicate 
parenchymatous cells—such as those described for Sigillaria 
spinosa —may have had an aerating function. 
Stem. 
The chief conducting system of the stem forms a continuous 
cylinder, consisting of xylem elements in the centre, and clothed 
within and without with phloem, pericycle and endodermis in the 
manner characteristic of a solenostele. There are in addition a few 
slender accessory steles, lying in the ground tissue enclosed by the 
solenostele. The ground tissue immediately surrounding the soleno¬ 
stele, on the inside and on the outside, is differentiated into a broad 
band of sclerenchyma. There is a third sclerenchymatous sheath 
round the exterior of the stem. The solenostele is roughly triangular 
in transverse section, following the general contour of the stem 
(Text-fig. 27). 
Petiole. 
The petiole shows a far more complicated vascular system than 
we should expect from the comparatively simple arrangement in the 
stem. 
1 Scott. “ Studies in Fossil Botany,” p. 227. 
