208 Worsdell.— The Vascular Structure of 
much larger than, often extremely large as compared with, 
the other intermediate bundles which run right through into 
the sterile portion without taking any part whatever in the 
conduction of substances to the sporangia. These latter 
bundles are always quite small, their xylem and phloem 
being but poorly developed. The lateral bundles, on the 
other hand, have a great development of centrifugal xylem 
and phloem, as would, from their function, be expected. For 
each one has to supply a large megasporangium, and this 
over a protracted period, during which the processes of 
sporangial development, fertilization, and embryonic growth 
must supervene, demanding a constant and adequate stream 
of nutritive substances through the tissues of these bundles. 
Hence is the difference in structure and development between 
the bundles of one and the same female sporophyll, and 
between these and the bundles of the male sporophyll, easily 
explained. These larger bundles very often have an element 
or two of centripetal xylem, while this tissue is usually absent 
in the smaller bundles, the large development of the bundle 
seeming to go hand in hand with the presence of centripetal 
xylem, as will be seen more clearly when the individual 
genera are treated of. 
The structure of the bundles in the thickened, expanded 
portion of the female sporophyll agrees essentially with that 
of the same region in the male organ. An exception to the 
general structure prevailing here is found in the bundles, or 
a certain number of them, which enter the megasporangium, 
and occur immediately below the base of the latter ; these 
bundles assume, in many cases, a perfectly concentric structure, 
which may or may not enclose a central pith, and may be 
large or quite minute in size ; others are not completely but 
only partially concentric, while others again are quite collateral 
in structure. All these kinds of structure may occur in the 
same group of bundles entering the sporangia. These con- 
centric strands I regard not as purely adaptive structures 
connected with the radial symmetry of the sporangium they 
supply, for though this idea might explain the structure of 
