Special M orphology and Classification. 329 
rows of smaller leaves. In both families a number of 
adventitious roots are produced on the under side of the 
stem facing the ground, and the stem contains an axial vas- 
cular bundle. In contrast to these two families, which are 
both terrestrial, 1s a third, the /soé/ee (Fig. 451), consisting 
of aquatic plants with a simple cylindrical but only slightly 
developed stem and elongated grass-like leaves. The single 
genus belonging to this family, /scéfes, is the only one at 
present known among Cryptogams in which the stem per- 
manently increases in thickness ; and this takes place by an 
annual formation of new masses of tissue round the cen- 
tral vascular bundle, the older ones gradually dying off on 
the outside. Since this process takes place almost entirely 
at two spots directly opposite to one another, the stem ul- 
timately has the form of a plate or disc (Fig. 451 11.). The 
axis of the stem is occupied in all the Lycopodiaceze by 
one or several vascular bundles separated from one another 
by intermediate parenchymatous fundamental tissue (Fig. | 
452). In the Selaginelleze the vascular bundle is connected 
with the cortex by a very loose spongy tissue, so that it 
appears to lie almost isolated in a cylinder filled with air 
and connected with the walls only here and there by 
parenchymatous cells. In the other families of the class a 
sheath of thick-walled fibriform cells arranged in several 
layers takes the place of this spongy tissue (Fig. 452 Ss). 
The structure of the vascular bundle itselfis always uniform. 
The xylem-portion consists of wider vascular cells in its 
inner, of narrower vascular cells in its outer part ; the bast- 
portion possesses vessels, fibres, and parenchymatous tissue. 
The axial vascular bundle sends out ramifications into the 
branches and leaves. | 
In all the Lycopodiacez the fructification takes the 
form of sporangia or capsules which burst open when ripe, 
seated in the axils of the leaves, and often, asin Lycopodium, 
constituting a peculiar spike-like receptacle. 
The families Selaginelleze and Isoétex have two kinds: 
