4 8 
L. A. Boodle. 
SECONDARY TRACHEIDES IN PSILOTUM. 
By L. A. Boodle. 
(Preliminary Account). 
ff^HE vegetative structure of Psilotum is in most respects well 
known from the works of Bertrand 1 and other authors. 
In Psilotum triquetrum the plant may be roughly described as 
consisting of a branched 2 subterranean rhizome and of vertical aerial 
shoots. In the vertical shoots, which bear scale-leaves, a trans¬ 
verse section of the stele contains an exarch polyarch xylem-mass, 
which has the form of a star enclosing a strand of sclerotic tissue. 
Towards the base of the aerial branch this structure becomes 
simplified by the gradual decrease in the amount of sclerenchyma 
present, until none is left, and the xylem then forms a solid strand, 
which has previously become reduced first to tetrarch then to 
triarch structure. Thus the vascular system of the lower part of 
the aerial shoot has a typical exarch protostele. The rhizome also 
is protostelic, containing an exarch triangular (triarch) or often 
band-shaped (usually diarch) mass of xylem or a small group of 
tracheides ( e.g . 5—6) with no very definite protoxylem. A certain 
amount of parenchyma sometimes three or more cells thick lies 
between the xylem and the sieve-tubes. 
The above description refers to primary tissue only, and 
nothing more than this appears to have been hitherto observed, to 
judge from the descriptions of Bertrand, Russow, 3 etc. 
In the lower part of the aerial shoot and in portions of rhizome 
directly connected with aerial shoots a certain number of tracheides 
were found amongst the parenchyma, which lies between the usually 
compact primary xylem and the ring of sieve-tubes. These 
tracheides generally include a few or several in course of develop¬ 
ment, these elements being incompletely lignified and sometimes 
containing protoplasm. Immature tracheides are to be found in parts 
of the plant which are probably several months old, e.g. at the base 
of a well developed aerial branch, and throughout the branch (2 cm. 
long), from which the former sprang, and in the parent stem of this 
again at 2 cm. below the point of insertion. 
1 Bertrand, Reckerches sur les Tmesipteridees, Archives 
botaniques du Nord de la France, I. (iSSi), p. 252. 
3 Both dichotomously and apparently monopodially. 
3 Bertrand, loc. cit.; Russow, Vergleich. Unters. 
