142 
MORPHOLOGY 
with Lycopodiales are (i) the vascular anatomy; (2) the solitary sporan- 
gium on the adaxial surface of the sporophyll; (3) the development of 
the sporangium from a transverse row of 
initial cells; (4) the trabeculae, which also 
appear in certain fossil Lycopodiales; (5) the 
ligule, which is present in all Lycopodiales 
except Lycopodiiim and Phylloglossum ; and 
(6) the gametophytes, which resemble closely 
those of Selaginella. The characters not in 
common with Lycopodiales are (i) the large 
multiciliate sperms; (2) the absence of a 
suspensor ; (3) the general habit ; and 
(4) the highly specialized leaves. 
(2) PSILOTALES 
This group of pteridophytes comprises two very 
small living genera : Psilotum (fig. 330), with two 
species occurring in the tropics of both hemispheres ; 
and Tmesipteris (fig. 325), with a single Australasian 
species (sometimes more species are recognized). 
These forms are intro- 
duced here, not to present 
their life histories, but to 
illustrate very briefly cer- 
tain stages in the evolution 
of pteridophytes. 
Throughout Lycopo- 
diales there appears a 
tendency to increase the 
output of spores produced FlG 32g _ Tmesip . 
FIGS. 325-328. Tmesip- by the sporophyte. The t cris: section of sporangia 
teris: 325, general habit, show- first and simplest method at early stage, showing 
ing branching leafy stems was by branching, thus between the two sporog- 
arising from a tree trunk (epi- multiplying strobili. There enous masses a sterile 
phytic) and bearing (near tip) was also an i ncr ease in the P late of cells which is 
the characteristic sporangia; size of sporangia . and to form the partition 
326-328, various views of the .,. , , r that divides a sporangium 
. this led, apparently for . 
paired sporangia (synangia). . . (single at the beginning) 
After PRITZEL (ENGLER and nut " tjve reasons, to the into two sporangia . _ 
PRANTL). development of sterilized After BowER 
plates through the spo- 
rangium, as in certain ancient lycopods ; and in Isoetes these plates almost divide 
the sporangium into chambers. 
In Psilotum and Tmesipteris there are two further stages of development in this 
