507 
Secondary Xylem in Psilotum. 
where the protoxylem is clear, it is evident that the differentiation of the 
primary xylem is centripetal in the rhizome and in the stock of the aerial 
stem. 
At the base of the stock of an aerial branch the xylem is often diarch, 
as in many of the larger branches of the rhizome, but when traced upwards 
(acropetally) the stele gradually increases in size and becomes triarch ; above 
the soil the exarch stele continues to enlarge, becoming successively tetrarch, 
pentarch, and so on, up to e. g. octarch, but, during this enlargement of the 
xylem, sclerenchyma (preceded by a little parenchyma) has appeared 
within it in a central position, and has increased to a large central group, 
so that the tracheides now form a hollow many-rayed exarch star enclosing 
this group of sclerotic tissue 1 . Figures illustrating the form or structure 
of the stele of the aerial stem are given by Brongniart (’37, PI. XI, Fig. i), 
Link (’42, Fasc. IV, Taf. V, Fig. i), Nageli (’58, PI. I, Fig. 3 ), Bertrand (’81, 
Fig. 174 , &c.), and Pritzel (’00, Fig. 383 a). 
So far all that has been said refers to the primary structure of the 
stem, and nothing besides this is to be seen either in Bertrand’s figures of 
the rhizome and aerial stem, or in the illustrations of the other authors 
referred to. In certain parts of the plant however, when examined at 
a sufficiently late stage, additional tracheides are found to be present. 
They will be described below as secondary, and the reasons for regarding 
them as such will be given later. 
In Fig. 1 , PI. XXXIII, which is a transverse section of the rhizome 
with primary structure only, one sees a zone of f parenchyma 2 ’ often three 
to four cells thick between the centrally placed group of xylem-elements 
and the zone of sieve-tubes 3 , the position of the latter being given at s. t. 
A similar zone of parenchyma is found in a corresponding position in the 
stock of the aerial shoot and also in the aerial shoot itself (though here 
often reduced opposite the protoxylem groups). The primary xylem is 
generally composed of a solid mass of tracheides, not interrupted by 
parenchyma, though this is not without exception, and the development is 
centripetal. 
An examination of the stock, of the lower part of the aerial shoot, and of 
some parts of the ordinary rhizome (in each case when old) showed additional 
1 Reduction of the stele through somewhat similar steps accompanies its decrease in size 
through successive branches of the aerial stem. 
2 Thin-walled considerably elongated elements. 
8 A careful examination of these elements was not made, but it may be mentioned that, in the 
presence of numerous granules clinging to the walls, they resemble the Fern-type, that they are 
present in the rhizome as well as in the aerial stem, although De Bary (’ 77 , p. 349) failed to find 
them in the former. In the rhizome and stock of the aerial stem they form an interrupted ring, 
single sieve-tubes and also tangential rows of two or three sieve-tubes being separated by parenchyma. 
In the aerial stem they may form a thicker zone. There are no special gaps opposite the protoxylems ; 
see Russow (’ 75 , p. 40), where he corrects the mistake as to their distribution shown by his earlier 
figure ( 72 , Taf. XI, Fig. 30). 
