178 
Ethel N. Thomas. 
The protoxylem groups of rootlets still enclosed in the parent 
tissues seem to be single (PI. IV., fig. 8). 
In the stout roots (PI. III., fig. 3 and PI. IV., fig. 7), as has been 
mentioned above, the three phloem groups are always bisected by a 
narrow, pointed, single protoxylem group, which alternates with the 
characteristic wide double protoxylem poles. As the phloem groups 
are shallow', this gives the large xylem body an almost circular 
appearance, although the triangular plan is quite discernible. The 
rounded contour of the xylem body, coupled with the comparative 
smallness of the phloem groups, and the presence of a great 
quantity of parenchyma in the xylem gives rise to a very stem-like 
appearance, recalling indeed the stem of Gleichenia. 
The only Fern root I have seen figured in the readily avail¬ 
able literature with which it can be even remotely compared, is 
that of Angiopteris evecta} This has a polyarch structure and is 
altogether planned on generous lines. It approaches the root of 
Acrostichum aureum in the disposition of its abundant parenchyma. 
The stout roots of the latter plant, however, bear a far more striking 
resemblance to the petiole of Lygodium japonicuiu -! 
We see then that in the character of the stele and of the 
cortex the roots of this Fern are exceptional and shew distinct 
resemblances to the stem-type. It will be remembered that 
Westermaier 3 has pointed out that the pneuniatopliores or air-storing 
roots of the mangroves proper have many stem characters. These 
organs are in fact intermediate in structure between true stems and 
roots. Thus the primary xylem is more or less centrifugal in develop¬ 
ment, and phloem may occur on the same radius. They are further 
provided with a pith and have no root-cap. 
Many pneumatophores have a peculiar development of the 
periderm for purposes of aeration. The phellogen forms layers of 
loose parenchymatous cells—“ aerenchyma ”—alternating with true 
cork layers. The phellogen arises in the cells of the outer cortex, 
thus being superficial in origin—another shoot-like character. 
1 have frequently noticed a very regular disposition of the cells 
of the outer cortex of the root of A. aureum , and have occasionally 
observed a distinct tangential w'all in some of the large rounded 
cells of a continuous layer. This appearance suggested the possi- 
1 R. Shove. Ann. of Bot., 1900. 
1 Boodle. Anatomy of the Schizaeaceae. Annals of Botany, 1901. 
3 Westermaier. Zur Kenntnisder Pneumatophoren. Botanische 
Untersuchungen im Anschluss an eine Tropenreise, Freiburg, 
1900, 
