PTERIDOPHYTA— HYDEOPTEEIDE^ 



145 



on stalks springing from the petioles of the leaves much as in 

 Ophioglossum ; and in Pilularia from the rhizome on the ventral 

 side of a leaf. 



In Salvinia and Azolla, the two genera of the Salviniacese, the 

 structure is a sorus of 



sporangia, covered in by Fm. 903. 



a cup -shaped indusium, 

 which differs from the 

 corresponding structure in 

 the ferns by being com- 

 posed of two layers of cells, 

 separated in Salvinia by 

 an air space {fig. 900, b), 

 and by completely closing 

 over the sorus. In Azolla 

 the walls of the upper part 

 of the indusium become 

 ligniiied. At the base of 

 the globular chamber so 

 formed there is a cellular 

 placenta, from which the 

 sporangia spring. Each 

 sorus consists either of 

 microsporangia or macro - 

 sporangia, but never con- 

 tains both. The nmnber 

 of microsporangia in a 

 sorus in botli genera is 

 considerable . The macro - 

 sporangia are less nume- 

 rous, the sorus of Salvinia 

 containing not more than 

 twenty -five, while that of 

 Azolla contains only a 

 single one. 



In the Marsileaceae 

 (Pilularia and Marsilea) 

 the sporangia are borne in 

 a complex structure kno^vn 

 as a sporocarp. This is 



a modified leaf-branch, as shown in fig. 901. It is an oval or 

 globular body with a very hard wall ; its interior is divided into 

 a number of chambers, each of which contains a sorus. The sori 



VOL. II. L 



Fig. 903. Pilularia globulifera. A. Natural 

 size. B. End of shoot (enlarged), s. Apex 

 of stem. b. Leaves, w. Roots. /. Sporocarps. 



