450 
VASCULAR CRYPTOGAMS. 
are the same, right and left being transposed. In each whorl the submerged leaf 
is the oldest, the one further from it of the two aerial leaves the second ; the nearer 
aerial leaf is the last formed. Each leaf arises from a cell of definite position, 
which becomes arched outwards (Fig. 316, B, L^, L^, and, becoming the apical cell 
of the leaf, forms a row of segments on each side. 
In the genus Azolla, which has been studied by Strasburger, the apical cell 
of the horizontal floating stem which curves upward near its growing end gives 
rise to a right and to a left row of segments, each of which is divided by a lateral 
longitudinal wall into a dorsal and a ventral half. Each of these halves is divided 
by a transverse wall into an acroscopic and a basiscopic portion, and each of these 
four cells is further divided into two by an oblique or a vertical longitudinal wall. 
The stem then consists (disregarding the subsequent divisions) of eight longitudinal 
rows of cells which have been formed from two rows of segments. The two 
dorsal rows remain sterile and form neither leaves nor buds. The two rows of 
leaves lie right and left on the dorsal aspect, and from the neighbouring rows 
Fig. 316. — Apex of the horizontal floating' stem of Salvinia (after Pringsheim) ; A ventral side, 5 left side, C transverse 
section of the long vegetative cone, 55 apical cell of the stem, y its last septum, iv submerged leaf, Z its lateral teetli, 
L L aerial leaves, h h hairs. 
of the ventral half, a little in front of or a little behind the leaves, arise the branches 
of the stem. Finally, the two inferior ventral rows bear the roots, each of which 
arises close to a bud and grows by means of a three-sided apical cell. If the 
leaves miarked in Fig. 315 a be regarded as the only ones present, the arrange- 
ment of the leaves in Azolla is approximately represented, with reference to which 
the buds and roots are placed in the manner above described. However, the 
arrangement of the leaves differs from this diagram in so far that, in Azolla, 
the leaves of the one row all arise from one cell of the acroscopic part of the 
segment, whereas those of the other all arise from one cell of the basiscopic 
part, in consequence of the position of the first leaf which always arises on the 
inner side of a branch and is directed towards the parent stem. Between any 
two leaves, which are placed alternately and in two rows, is an internode of the 
length of half a segment, one side of the internode being formed from the basiscopic, 
the other from the acroscopic half of a segment. 
In Marsilia the apical cell of the embryo is so placed that dorsal and ventral 
