332 Bower . — On the Structure of the Axis 
represents the space which was occupied by the delicate 
tissue of the phloem. 
Similarly, cavities are found in the stems of the various 
species of Lepidodendron. The bulky cortex in these is usually 
distinguishable into three bands, of which the middle band is 
rarely preserved ; in some species there is still very little 
detailed information as to the nature of this tissue ; and even 
in most of those cases where it has been recognized as a con- 
tinuous tissue the state of preservation does not appear to 
have been sufficiently good to allow of detailed description 1 . 
Suggestions have been made in certain cases that this middle 
layer consisted of spongy parenchyma, and so forth ; but till 
very recently little was positively known of the parts which 
are so frequently missing. The tissues immediately outside 
the xylem, presumably of the nature of phloem and protective 
sheaths, together with the innermost band of cortex, are also 
commonly absent 2 , or imperfectly preserved 3 . Quite recently, 
however, M. Hovelacque has published a most exhaustive 
description of the structure of the stem of Lepidodendron seta - 
ginoides , Sternb. 4 He has studied the entire succession of 
tissues which compose the stem ; he finds the tissues of this 
species, when well preserved, to be a continuous sequence, 
without discontinuity or large lacunae, and distinguishes three 
chief zones of the tissues outside the central vascular stele ; 
of these his inner cortex (ecorce interne) presents the most 
interesting characters for comparison with the results to be 
described below. It is composed of (i) the protective sheath, 
which is simply the innermost layer of cells ; (2) the inner 
zone, consisting of thick-walled cells ; (3) the middle zone, 
often destroyed, consisting of thin-walled cells with inter- 
cellular spaces; (4) the outer zone of thick-walled cells. 
The zones (2), (3) and (4) appear to correspond essentially to 
1 Compare Solms’ figure of L. selaginoides , Will., Fossil Botany, Fig. 23, p. 221. 
Engl. ed. : also Figures by Williamson, Phil. Trans. Part II, 1881, &c. 
2 Renault, Cours de Botanique fossile, vol. ii, PI. 4, Fig. 3, and PI. 10, 
Figs. 4, 5. 
3 Williamson, loc. cit., PI. 51, Fig. 10, &c. 
i Mem. d. 1 . Soc. Linn, de Normandie, vol. xxii, Fasc. 1. 
