392 Chandler . — On the Arrangement of the Vascular Strands 
Structure of the Young Stolons . 
The structure of the young stolon is essentially the same as that of 
the stem at the level at which its junction with the latter is effected. The 
xylem contains a large amount of parenchyma, and possesses three or four 
peripheral protoxylem groups. A short distance behind the apex the 
latter are well marked, the large thin-walled tracheae of the metaxylem 
occupying the central portion of the strand. The phloem, with fairly 
conspicuous sieve-tubes, which show a tendency to arrange themselves in 
groups as in the stem, forms a complete sheath round the xylem. An 
older stolon shows a more or less sclerotic fundamental-tissue contrasting 
sharply with the small-celled epidermis. The well-marked endodermis, 
the cells of which are in seriation with those of the pericycle, is surrounded 
by two or three layers of extremely thick-walled ground-tissue cells. The 
xylem is still largely parenchymatous, but the tracheae are in three main 
groups, each with peripheral protoxylem elements. The phloem and peri- 
cycle call for no special description, except that the latter does not vary in 
thickness to any great extent as in the stem. 
Nephrodium setigerum, Baker. 
The earlier transitional changes of Nephrodium have been described 
by Leclerc du Sablon 1 in N. molle. Only very young plants were 
examined, however, and the writer has been able to make a more complete 
study of N. setigerum. The root is diarch, and its transition to the 
solid protostele of the young stem is, on the whole, normal. The phloem, 
however, takes a considerably longer time than usual to completely surround 
the xylem rod, and generally the phloem sheath is not complete until one 
or two parenchyma cells have appeared in the centre of the xylem. Sieve- 
tubes are very quickly differentiated in the pith, taking up a central 
position, so that we have internal phloem present before the exit of the 
first leaf-trace. It is a point of some interest that, in two or three of the 
plants examined, isolated tracheides were found in the central phloem, 
thus emphasizing the essentially vascular nature of the cauline strand as 
a whole at this level (Fig. 202). The first leaf- and root-traces are formed 
at approximately the same level, separated by about ioo°. 
The leaf-trace is of the normal concentric type, the xylem being a 
slightly curved band, and the shallow gap, which is occupied by the phloem 
of the pith, is merely bridged by the endodermis. The leaf-gap is hardly 
closed (in one or two cases it was actually not closed) when the next leaf- 
trace is formed exactly opposite the last, accompanied by two roots. The 
usual occurrence of a single root for each leaf does not seem to hold in 
Loc. cit. 
