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 

 atrial leaf is the last formed. Each leaf arises from a cell of definite position, 

 which becomes arched outwards (Fig. 316, ^, Z^, 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 oi Salvinia (after Pringsheim) ; A ventral side, 5 left side, C transverse 

 section of the long vegetative cone, ^S apical cell of the stem, y its last septum, -w submerged leaf, Z its lateral teeth, 

 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 marked L^ in Fig. 315 « 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 



