418 SPHENOPHYLLALES. B. PSILOTACEAE 
The same is the character of the more superficial cells of the sporogenous 
mass (Fig. 232 D), so that it is almost impossible to recognise the limit 
between the tissue of the wall and of the sporogenous mass: the superficial 
portions of the latter become disorganised without the formation of spores, 
but there is no clearly defined tapetum. Such is also the fate of a 
considerable proportion of the more central cells: for as the synangium 
develops, irregular groups of cells of the sporogenous masses assume 
dense granular contents, and subdivide, while the others remain paler, 
with more watery contents, and do not divide: the former undergo the 
final tetrad-division and form spores, while the latter become disorganised. 
The actual state of partial disorganisation is shown in Fig. 232 E: thus a 
partial sterilisation of cells of the sporogenous tissue, essentially similar to 
that in Eguisetum, is seen also in Psz/otwm, and, as above stated, it occurs 
also in Zmeszpteris. 
In both genera the lines of dehiscence at maturity are defined 
structurally. In Pst/otwm the lines radiate from the centre,’ and it has 
been seen that a similar condition may be recognised in the sporangio- 
phores of Sphenophyllum majus. Thus, in the broad outlines of 
structure, in function, and in position the sporangiophore of the, Psilotaceae 
is the correlative of that in the Sphenophylleae. It remains to see how 
far there is a correspondence also in the anatomical structure of these 
plants. 
ANATOMY. — 
The internal structure of the Psilotaceae is relatively simple, in 
accordance with their outward form. The axis is traversed by a stele, 
which is fundamentally of the protostelic type, and limited by an 
endodermis which is more definite in Psz/otum than in Zmesipteris. The 
broad cortex which surrounds the stele is variously differentiated in the 
rhizome and in the aerial shoot: in the former it consists of starchy 
parenchyma, with endophytic mycorhiza in the outer layers, while the 
superficial cells grow out into rhizoids: in the aerial shoot the stele is 
surrounded successively by thin-walled parenchyma, sclerenchyma, and 
assimilating tissue, while peripherally is an epidermis with stomata. Such 
characters, however, present little that is of comparative value; it is in 
the vascular tissue that a better basis for comparison is found. 
The structure of the stele in Psz/otwm! varies according to the position 
and size of the part: in the rhizome there is often no protoxylem, but 
the xylem consists of a somewhat irregular, and exiguous group of 
scalariform tracheides, surrounded by an ill-developed phloem, while peri- 
pherally there is a definite endodermis. At the base of an aerial shoot 
the xylem increases in bulk, with interspersed parenchyma cells, and 
1 These statements are based partly on personal observations, but also on the writings 
of Bertrand (Avch. bot. du Nord., i.), of Boodle (Aun. of Bot., xviii., p. 505), and of 
Miss Ford (Azz. of Bot., xviii, p. 589). 
