546 Brehner . — On the Anatomy of 
polyarch actinostele of the aerial roots of Angiopteris. The 
subterranean roots only differ in the smaller number of the 
alternating groups of xylem and phloem, and slightly in 
texture of the cortex. The structure of the oligarch central 
cylinder of Angiopteris is practically identical with that of 
the fossil genus Psaronius , which is considered to have 
Marattiacean affinities 1 . It is unnecessary here to go into 
the structure of the actinostele of these plants since it has 
been so recently illustrated and described, nor is it desirable 
to give details of the varying number of the alternating 
protoxylem and phloem groups in the different genera. There 
remains, therefore, only to be mentioned one or two slight 
anatomical differences, which are of more or less diagnostic 
value. As noted by Harting, in the celebrated de Vriese and 
Harting monograph, there are very thick- walled sclerotic cells 
sparingly distributed in the cortex of the subterranean roots, 
and this seems to be generically characteristic. In Danaea 
Kuhn found a selerenchymatous pith 2 in the various species 
examined by him, but curiously enough a few specimens of 
Danaea alata f Sm., did not have it. The thick-walled fibres 
constituting this pith are differentiated early (cf. Fig. 26, 
PL XXIII). Danaea also has a continuous ring of scleren- 
chyma fibres two or three cells thick lying a few cell-layers 
below the epidermis. The protoxylem may abut directly on 
the endodermis in the older as well as in the primary and 
earlier cauline roots. In the figure this is seen to be the 
case with the protoxylem adjacent to the endodermal cell 
marked en. 
In the article Marattiaceae of Engler and Prantl’s Natiir- 
lichen Pflanzenfamilien, the development of the tracheae of the 
roots is said to be centrifugal 3 . After this was noticed there 
was no time to go into the matter practically, but the order 
of lignification of the tracheids is undoubtedly from without 
1 Cf. D. H. Scott, Studies in Fossil Botany, p. 268 et seq., and Farmer and 
Hill, loc. cit., p. 268 et seq. 
2 R. Kuhn, Ueber den anatomischen Bau von Danaea, Flora, 1890. 
3 Loc. cit., p. 430. 
