FILICINE^. 



449 



The embryo of Salvima, as long as it is enclosed in the prothallium, forms, as 

 we have seen, the segments of its apical cell alternately above and below; but when 

 the apex of the stem is exposed in consequence of its elongation, a torsion takes 

 place to the extent of about 90°, so that the two rows of alternate segments of the 

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

 meister in Pteris aquilina. The first* leaf is the scutiform leaf mentioned above, 

 which is placed medio-dorsaily ; then follow a second and third aerial leaf standing 

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

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

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

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

 quite fiat laminae and spring from the dorsal side, touching the water only with their 

 under surface (Fig. 319). These three-leaved whorls alternate, and thus form two 

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



Fig. 315 a.— A the vegetative cone of the stem oi Salvinia nutans, represented diagrammatically and looked at from 

 above; xx projection of the plane which divides it vertically into a right and left half; the segments are indicated 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 w; w the first-formed submerged leaf; 

 L\ the aerial leaf formed next ; 7.2 the second atrial leaf of the same whorl formed last of all between the two first (after 

 Pringsheim). 



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

 are indicated in Fig. 315 «. The node of the stem which produces a whorl of 

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

 vegetative cone, which in its length (or height) corresponds to a half-segment, while 

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

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

 different ages ; in Fig. 315 a an internode is formed of the segment H on the right 

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

 younger segment, J, on the left side ; the next internode is the product of the whole 

 of the left segment, Z, and of the two halves of K and M lying to the right ; the 

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

 hand, of the anterior half of the left older segment J and of the posterior half of the 

 right younger segment K\ in the preceding and succeeding node the relationships 



Gg 



