RHIZOCARPEM. 



39^ 



which becomes arched outwards (Fig. 291, 7y,Zj, ZJ, and, becoming the apical cell 

 of the leaf, forms a row of segments on each side. 



I-'K.. =01— Apex of the horizontal floating stem of Salvinia (after Pringsheim) ; A ventral side, A' dorsal side, 

 C transverse section of the long vegetative cone, .VS apical cell of the stem, j* its last septum, iv submerged leaf, 

 Z its lateral teeth, /. /. aerial leaves, h ft hairs. 



In Marsilea the apical cell of the embryo is so placed that dorsal and ventral 

 segments in two rows are at first formed from it by walls inclined upwards and 

 downwards ; the dorsal median leaf also proceeds from the first dorsal segment. 

 But a diff"ercnt arrangement is soon produced as the plant increases in strength; 

 the apical cell of the stem forms segments arranged in three rows with a \ diver- 

 gence, and in such a manner that one row of segments comes to lie below (ven- 

 trally), while the two other rows form the dorsal side of the stem ; the ventral side 

 of the stem forms roots in strictly acropetal succession, the youngest being found 

 near the apex of the stem. On the dorsal side of the stem the leaves arise in two 

 alternating rows, some of the dorsal segments remaining at the same time sterile 

 and serving for the formation of internodes \ The first leaf of the young plant, 

 arranged on the median line and without a lamina, is followed, in the biseriate 

 arrangement which now results, by a number of young leaves with a short stalk 

 and a lamina at first entire but afterwards divided into two and four lobes ; normal 

 leaves circinate in their development are then for the first time formed with a long 

 stalk and a quaternate lamina. In the processes which have now been described, 

 Pilularia agrees, according to Hanstein's observations, with Marsilea, except that all 

 the leaves remain destitute of a lamina (Fig. 293); they are long, conical^ filiform, 

 and at first rolled up spirally forwards. 



The Branching of Rhizocarps is similar to that of Ferns. Pringsheim states 

 that in Salvinia terminal branching never occurs ; new shoots arise, on the contrary, 

 exclusively from the basal part of the submerged leaves, each leaf of this description 

 forming a shoot on the side which faces the nearer aerial leaf; every branch pro- 

 duces at once a trimerous whorl of leaves. The branching of the Marsileaceae has 

 been termed by Hanstein axillary, a designation with which, however, I am unable to 

 agree. The lateral shoots have altogether the appearance of springing from the 



* Compare the corresponding processes in Radula and other foliose Jungermannieae, p. 309. 



