ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 907 
tissue (see Plate 74, photograph 14, Plate 80, figs. 23 and 25). Manitestly, the amount 
of secondary growth in thickness in the axis of the strobilus was relatively small 
and inconstant, just as we know to be the case in the peduncles of Dicotyledons at 
the present day. The formation of secondary wood in many cases, however, is 
amply sufficient to prove that the mode of growth was essentially similar to that 
of Calamites. 
The cortex of the axis of the strobilus needs no detailed description. Its outer 
layers, which in most cases are alone preserved, consist of thick-walled elongated 
cells, often prosenchymatous in form. When portions of the inner cortex are 
preserved, its cells are similar to those of the more external layers in form, but have 
thin walls. It is possible that this inner layer should be referred to the pericycle, 
but we cannot undertake to determine the limits of these tissues with accuracy (see 
Plate 73, photographs 10-13, Plate 74, photograph 14; Plate 81, fig. 27, where only 
the inner layers are shown). 
The epidermis consists of small cells, often with rather thick walls; we were not 
able to prove the presence of stomata, but it is quite possible that they existed. 
The specimens rarely have their epidermis so well preserved as the more internal 
tissues. 
3. Appendages of the Axis. 
A. The Bracts. 
The general arrangement of the whorled bracts has been already described. The 
coherent part of the whorl, or disc, is traversed by vascular bundles equal in 
number to the free limbs of the bracts, into each of which one of the bundles runs. 
out (see Plate 73, photographs 11 and 12; also Plate 81, figs. 30 and 32). Asa 
rule, only the small xylem-strand of the foliar bundle is preserved ; this lies within 
an empty space, from which the soft tissue has perished. Sometimes a part of this 
soft tissue is preserved, and then we find it on the lower side of the xylem. It thus 
appears that the bundles were normally collateral, as we should expect. 
The mesophyll of the coherent disc usually shows a marked differentiation. 
Towards the upper surface it consists chiefly of thick-walled sclerenchymatous 
fibres. Its lower portion is composed of more parenchymatous cells, with wider 
lumina, and usually thinner walls. Next the lower epidermis we find a small-celled 
hypodermal layer. The epidermis on both surfaces has small cells, and is seldom 
very well preserved, so that here also the question of stomata must be left open. 
There are many variations in detail in different specimens ; sometimes the whole 
tissue of the coherent region is comparatively thin-walled (see Plate 81, fig. 30) ; 
sometimes, on the other hand, the parenchyma, as well as the fibrous tissue, is 
considerably thickened (see Plate 73, photographs 10 and 11). In some cases a strand 
of specially thick-walled sclerenchyma follows the course of each vascular bundle on 
its upper side (photograph 11). Certain large parenchymatous cells towards the 
