94 
M. C. St opes on 
Cycads. All the bundles in these sections, however, are essentially 
similar, and, with one exception, are running practically parallel, as 
is shewn by the distances between each pair of bundles in the 
different sections of the series. 
The cells of the xylem-parenchyma are large, thin-walled, and 
somewhat oblong in shape ; sometimes, as in phot. 3, a single layer 
of large size fills the space between the protoxylems and the inner 
sheath, or as in figure 5, there may be two or even three layers of 
smaller cells between the two sets of hard tissue. The protoxylem 
is separated from the phloem by two or three large and irregular 
parenchyma cells. In longitudinal section the xylem-parenchyma 
is elongated and thin-walled, the transverse walls being slightly 
thicker than the others. 
The phloem consists of small square elements, a few of which 
have structures suggestive of protoplasmic contents and nuclei ; in 
longitudinal section the phloem shews delicate oblique walls on 
which we have not been able to detect sieve-plates. 
The bundle sheath is well developed and is composed of two 
types of tissue, forming an inner and outer sheath. The outer 
sheath consists of two or three layers of large cells which touch 
the sclerenchyma above and below the bundle (phot. 3 and 4, fig. 5). 
They are more largely developed on the upper side and sometimes 
the cells between the xylem and upper sclerenchyma are of con¬ 
siderable size. At the sides the large cells of the outer sheath are 
adjacent to the palisade and spongy parenchyma, from which they 
are quite distinct, but in the middle, i.e., on the flanks of the bundle, 
the outer sheath-tissue sometimes shews a tendency to become 
merged with the adjacent elongated middle tissue of the leaf. 
The inner sheath lies within the outer, closely fitted to it, the cells 
of the inner sheath form an arc round the phloem and attach them¬ 
selves to the flanks of the most centripetal xylem-elements, where 
five or six of them frequently form a group on either side. The 
cells of the inner sheath are small and exceedingly thick-walled, 
recalling sclerenchyma in transverse section (phot. 3 and 4., fig. 5, 
55 ). In longitudinal section (fig. 6) they are seen to be long and 
straight, their thick walls pitted with a single row of very well- 
marked bordered pits; they appear much more like true xylem 
than the transfusion-tissue of recent Cycads, to which we will 
compare them later. The outer sheath, on the other hand, consists 
of larger cells with thinner walls, also pitted with bordered pits, 
although in a more irregular fashion (fig. 6, s.) ; those which abut 
