;90 



VASCULAR CRYPTOGAMS. 



pical cell lie right and left, a peculiarity which has also been observed by Hof- 

 leister in Pieris aquilina. The first leaf is the scutiform leaf mentioned above, 

 :hich is placed medio-dorsally ; then follow a second and third aerial leaf standing 

 ingly, after which the definite verticillate arrangement of the leaves at length 

 ommences at the fourth node ; each whorl thereafter consists of a submerged leaf 

 pringing on the ventral side (right or left), which at once branches, and forms a 

 ift of long filaments hanging down into the water; while two other leaves have 

 uite flat laminae and spring from the dorsal side, touching the water only w4th their 

 nder surface (Fig. 295). These three-leaved whorls alternate, and thus form two 

 ows of ventral submerged, and four rows of dorsal aerial leaves. Their succession 

 1 age in the whorl, and the position of the wdiorls (antidromal among themselves) 

 5 indicated in Fig. 290 a. The node of the stem which produces a w^horl of 

 iaves, is, as was shown by Pringsheim, formed of a transverse disc of the long 



Fig. iiyia.—A the vegetative cone of the stein of Salvinia natans, regarded diagrammatically and looked at 

 from above ; xx projection of the plane wliich divides it vertically into a right and left half; the segments are indi- 

 cated by stronger outlines, their divisions by weaker lines ; the succession of the segments is denoted by the letters 

 F — P ; B diagram of the stem with three whorls of leaves, its ventral side indicated by vv; w the first-formed 

 submerged leaf ; L\ the aerial leaf formed next ; Z.2 the second aerial leaf of the same whorl formed last of all between 

 the two first (after Pringsheim). 



'egetative cone, w^hich, in its length (or height) corresponds to a half-segment, while 

 lach internode corresponds to the whole height of a segment. Each nodal disc, as 

 veil as each internode, consists of cells of the right and left row of segments of 

 lifferent ages; in Fig. 290 a an internode is formed of the segment H on the right 

 ide, of the anterior half of the older segment, G, and of the posterior half of the 

 'ounger segment, /, on the left side ; the next internode is the product of the whole 

 )f the left segment, L and of the tw^o halves of K and H lying to the right ; the 

 ntermediate nodal disc which forms the leaves w, Z^, L^ consists, on the other 

 land, of the anterior half of the left older segment / and of the posterior half of the 

 ight younger segment K ; in the preceding and succeeding node the relationships 

 ire the same, right and left being transposed. In each whorl the submerged leaf 

 s the oldest, the one further from it of the two aerial leaves the second ; the nearer 

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



