FILICINEAE. HETEROSPOROUS FILICINEAE. 



231 



merged in the formation of the archegonium is shown both by the history of de- 

 velopment, and by the fact that, when the oosphere in the single archegonium re- 

 mains unfertilised, the prothallium continues to develope and becomes a relatively 

 large body which contains chlorophyll, and has root-hairs proceeding from it. The 

 further growth of the prothallium ruptures the epidermal layers of the papilla above, 

 and its dorsal surface emerges into the free space known as the funnel beneath the 

 thick outer membranes of the macrospore; subsequently the wall or diaphragm 

 which separates the prothallium from the rest of the macrospore becomes arched 

 convexly outwards and thrusts the prothallium still further out of the macrospore. 



In Salvinia also a small meniscus-shaped cell is cut off from the rest of the 



FIG. 184. Sal'uinia natans. A longitudinal section through macrospore, prothallium and embryo in the median 

 line of the prothallium ; a cell-layer of the sporangium, b episporium formed of hardened mucilage, c coat of the spore 

 with its continuation e, d the diaphragm separating the prothallium from the spore-cavity, pr prothallium already 

 broken through by the embryo, m neck of archegonium, /, II the two first leaves of the embryo, v the apex of its 

 stem, s the scutiform leaf (cotyledon). B an older germ-plant with the spore sf and the prothallium pr ; a the caudicle, 

 * the scutiform leaf, /, // first and second single leaves, LL' aerial leaves of the first whorl, w its submerged leaf. 

 After Pringsheim. A magn. about 70, B 20 times. 



spore, and from this cell the prothallium proceeds 1 . But in Salvinia the prothallium 

 attains a much larger size than in the other two genera ; it contains a large amount of 

 chlorophyll, and produces several or even many archegonia in definite positions. 

 After it has burst through the membranes of the papilla, it is seen from above as a 

 three-sided body between the three lobes of the exosporium ; one of these sides is the 

 anterior side, the two posterior sides meet behind in an angle ; a line from this angle 

 to the middle of the anterior side passes along the raised saddle-like back of the 

 prothallium and is called the median line ; the anterior side rises higher than the 

 back, and the two angles formed by its junction with the posterior sides grow out 

 subsequently into long wing-like processes which hang down by the side of the 

 macrospore. The first archegonium appears on the median line immediately behind 

 the growing anterior side of the prothallium ; then two more archegonia are always 



1 See further in Prantl, Zur Entw.-Gesch. d. Prothall. von Salvinia natans (Bot. Ztg. 1879, 

 p. 425). 



