PTER1DOPHYTA. 217 



grow out through this slime and also through the wall of the spor- 

 angium (Fig. 214), and it thus terminates in a relatively long 

 cell. 



In Marsilia the microspores are set free from the microsporan- 

 gium, and the prothallia, with the antheridia, remain in them 

 until the spermatozoids are liberated. The latter are spirally- 

 twisted threads. 



The MACROSPORES, on germination, give rise to a very reduced pro- 

 thallium, which in Salvinia bears 3 archegonia ; but, if these are 

 not fertilised, the prothallium may continue to grow and become a 

 fairly large, green body with several archegonia (Fig. 215 A, B). 

 In Marsilia the prothallium is still more reduced, it is enclosed in 

 the macrospore, and only bears one archegonium. The archegonia 



FiG. 21G. Salvinia natan?. A An archegonium, unripe, teen in longitudinal section: 

 h the neck-cells; k the neck-canal-cclls ; c the central cell. B An open atchegonium of 

 which the neck-cells have separated off. C An open, old archegoniuin seen from the top. 



are similar in structure to those of the Ferns, but are smaller, and 

 sunk more deeply in the tissue of the prothallium. 



The asexual generation is developed from the fertilised 

 egg-cell. It is a dorsiventral, horizontal shoot. In Salvinia it bears 

 at first a shield-like leaf, the scutiform leaf (Fig. 215 (7, a), which 

 is succeeded by the ordinary foliage-leaves. The young plants 

 of Marsilia, likewise, have less perfect leaves in the very early 

 stage. 



The formation of the sporangium is the same as in the Lepto- 

 sporangiate Ferns. (The 16 spore-mother-cells originate from one 

 central, tetrahedric archesporium.) 



The Hydropteridese are divided into 2 orders, the chief differ- 

 ences between them being found in the asexual generation. 



