ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 693 
entire absence of phloém, while between them four distinct bands of phloém (ph) 
are shown. Thus the usual alternation, characteristic of roots, is maintained. 
Within each of the four bands of phloém growth in thickness has commenced, and 
from three to four layers of secondary wood have already been formed in this position. 
The rootlet shown in Plate 15, photograph 5, is essentially similar, but here the 
wood has already attained a somewhat greater thickness.* 
We now come to the subject of pericycle and endodermis. The rootlet shown in 
Plate 16, fig. 4, has four rows of cells between the phloém and the beginning of the 
lacunar zone. The two outer of these layers have intercellular spaces, and evidently 
form part of the inner cortex. The next layer, towards the interior (en, in fig. 4), 
consists of closely-fitting cells, and a slight thickening of their radial walls can be 
made out. This was almost certainly the functional endodermis. The innermost 
layer, abutting on the phloém and protoxylem, consists of thin-walled cells. Now it 
is remarkable that these four layers of cells are arranged, on the whole, in radial 
series. The same is the case in the rootlet shown in Plate 15, photograph 5. The 
important point is that the layers which occupy the respective positions of pericycle 
and endodermis fit on to each other as if they had had a common origin. Traces of the 
same arrangement are found in other specimens, both rootlets and larger roots.t 
In some of the latter the thin-walled layer is two cells thick. Outside this comes 
the functional endodermis, and sometimes all three layers are in radial seriation. 
Too much stress must not be laid on these facts, which are not absolutely constant. 
At the same time they are not without interest, for we know that in the roots 
of Hquisetwm there is no pericycle, but a double endodermis, in which the outer 
layer alone has the typical endodermal structure, while the inner layer is thin- 
walled, and gives rise to the rootlets.{ If the indications above mentioned are to be 
trusted it would appear that the same may have held good for the roots of Calamutes, 
in which case we should have to speak of the thin-walled layer immediately sur- 
rounding the stele, not as pericycle, but as inner endodermis. 
The cortex of the rootlets does not differ from that of the larger roots. We find 
the same three zones, of which the middle one is lacunar (see photograph 5, and 
figs. 4, 5, 7, and 8). Occasionally we find cells in the cortex which have rather 
thicker walls than their neighbours, and dark carbonised contents (see fig. 4). These 
may very probably have been secretory sacs of some kind. Similar cells also occur 
in the cortex of the largest roots (see fig. 11). 
Attention has already been called to the occasional presence of irregular rounded 
cells within the lacunz. This peculiarity appears both in the medullate roots and in 
* Of. “Organization,” Part XI, Plate 30, fig. 5, which represents a similar rootlet at a more 
advanced stage of development. 
+ As for example in C.N. 1308. 
+ See, for example, Van Tiscuem et Dovwtot, “ Origine des membres endogénes,” 1889, p. 394, 
Plate 27, figs, 413-416. 
