On the Occurrence of Secondary Xylem in Psilotum 1 . 
BY 
L. A. BOODLE, F.L.S. 
With Plate XXXIII and seven Figures in the Text. 
N examination of the structure of Psilottim and Tmesipteris was begun 
some years ago, at the suggestion of Dr. D. H. Scott, F.R.S., in 
connexion with the hypothesis put forward by him regarding these genera, 
viz. that of their affinity with the Sphenophylleae 2 among fossil plants ; 
the object being to learn what additional evidence a re-examination of the 
anatomy might afford in relation to this view. It was intended to publish 
the results in a joint paper, but, as this has been postponed, some observa- 
tions on Psilotum triquetrum , Sw., have been completed and are described 
separately in the present paper 3 . 
A few remarks must first be made on the external characters of the 
stem of Psilotum triquetrum . There are no roots, but the different 
branches of the stem may be classed under aerial parts and subterranean 
parts 4 . The aerial shoots are upright, repeatedly forked, and bear scale- 
leaves, and, in the upper region, sporophylls with synangia ; most of the 
subterranean branches have no scale-leaves, and are densely covered with 
rhizoids. Among the subterranean branches three types have been 
distinguished 5 , viz. (i) those completely covered with rhizoids and with 
only a terminal growing apex ; (2) those similar to (1) except that they 
have laterally placed arrested apices besides the terminal growing-point, 
the existence of these apices being revealed by the absence of rhizoids on 
their surfaces 6 ; (3) those in which the rhizoid-covering is reduced, and, on 
parts formed by further growth, is entirely absent, lateral apices being also 
present on these branches as well as small scale-leaves, and the direction of 
1 From the Jodrell Laboratory, Royal Botanic Gardens, Kew. 
2 Scott (’00), p. 499. 
3 A preliminary account has been given in the New Phytologist, vol. iii, 1904, p. 48. 
4 This is only a rough criterion. There appears not to be a constant relation between the 
level of the soil and a definite structural region of the stem. 
5 See Solms-Laubach (’84, p. 156), Nageli and Leitgeb (’68, p. 147), and Pritzel (’00, p. 612). 
6 Later they may grow out as branches. 
[Annals of Botany, Vol. XVIII. No. LXXI. July, 1904.] 
