Ford,— The Anatomy of P silo l am triquetrum, 599 
layers next to the endodermis have deeply coloured brown walls. Passing 
gradually up the stem, it is seen that the superficial hairs have disappeared, 
and the whole thin-walled cortex is gradually replaced by the brown- 
walled tissue. The xylem increases in amount, parenchymatous cells 
may be present scattered irregularly amongst the tracheids, but the 
protoxylem is often hard to determine, although further up the stem 
two and then three or more groups make their appearance. The 
secondary tracheids, which were first observed and have recently been 
described by Boodle 1 , appear in this region. Higher up, the stele passes 
through an interesting stage (Figs. 13 and 14 ). The amount of xylem 
increases in extent, the three or four protoxylem-groups can be readily 
distinguished, and the parenchyma, instead of being scattered amongst 
the tracheids, lies in the centre as a more or less distinct pith (pd). In 
Fig. 13 it is seen that one tracheid (t) alone is left in the centre, whilst 
the xylem forms a ring broken only at one point (g). Further up again, 
the xylem forms for a short time a complete, unbroken ring round the 
central pith. This tissue is composed of nucleated elements, with thin 
areas on their walls, but no sieve-tubes have been observed, nor has any 
internal endodermis been detected. A few sections higher up, the first of 
the central fibres makes its appearance (Fig. 14 ,/) in the pith, the number 
then gradually increases, until the typical central mass is formed, 
surrounded by the xylem with its radiating points of protoxylem. The 
cortex gradually changes from the uniform mass of brown-walled cells 
to the usual arrangement of parenchymatous and sclerenchymatous zones 
found in the aerial stem. 
The Reproductive Organs. 
The nature of the sporangial apparatus in the Psilotaceae has given 
rise in the past to much controversy. The researches and results of 
Professor Bower 2 seem, however, to be conclusive in ranking the 
sporangium itself, not as a terminal outgrowth of a reduced axis which 
bears two leaves, but as an outgrowth from the sporangial leaf, which 
is bi-lobed possibly for a protective purpose. The whole sporangiophore 
is then a single foliar member. Investigations on the origin of the 
sporangium during the work on this paper have led to similar conclusions. 
With the exception of the vegetative reproduction by means of 
bulbils, described by Solms-Laubach 3 , and the prothallus found and 
described by Lang 4 as possibly belonging to Psilotum , nothing is known 
of the development of either Psilotum or Tmesipteris. Attempts have 
1 Boodle (’04), p. 499. 2 Bower (’94), loc. cit. 
3 Solms-Laubach (’84), loc. cit. 139. 
4 Lang (’01), p. 405. See also Dr. Lang’s paper in the present number of the Annals of Botany, 
‘ On a Prothallus provisionally referred to Psilotum.' 
