142 MANUAL OF BOTANY 



to the sclerenchyma, which is never very prorainent and is in 

 some cases absent. Gum passages of probably schizogenous 

 origin are fomid m many parts. 



The sporangia are arranged in sori, but the several sporangia 

 of each sorus are coherent instead of being developed separately. 

 They thus form a body, at first sight suggesting a compound 

 sporangium. This is known as a synangium. They are filled 

 with numerous spores and dehisce when ripe by a slit or an 

 apical pore (fig. 899j. 



The gametoph;yi;e is a flattened green prothallium, much 

 like that of the true ferns. It has a cushion m the centre, on 

 which both antheridia and archegonia are borne. It grows by 

 an apical cell. The sexual organs have the same structure as 

 in the ferns. 



Section 3. — Heterosporous Leptosporangiate Ferns. 

 {Rliizocarps or Hijdropteridecs.) 



This group, which has much in common with the isosporous 

 ferns, introduces a difference which becomes more and more 

 important as we go higher and higher in the scale. The plants 

 bear spores of two kinds, the microspores and the macrospores 

 or megaspores. Each of these in turn produces a special form 

 of gametophyte, and those which are derived from the macro- 

 spores never become free from the spore, being largely developed 

 in its interior. As we pass upwards fi'om this point this 

 peculiarity becomes more and more marked, until we find the 

 prothallium always completely endosporous. The great im- 

 portance of this is seen in that it leads ultimately to the 

 production of the body kno^\Ti as the seed, which is the dis- 

 tinguishing feature of Phanerogamic plants. 



The Hydropteridese or PJiizocarps were till comparatively 

 recent times considered as a separate group. It is usual now to 

 include them with the Ferns, to which they show considerable 

 resemblance. They are all of aquatic habit, and are hence 

 named HydropteridecE . The group comprises four genera, 

 Salvinia, Azolla, Pilularia, and Marsilea, which according to 

 the arrangement of their sporangia are divided into the two orders 

 SalviniacecB and MarsUeacce. Salvinia and Azolla float freel}' 

 upon the surface of water ; each has a horizontal rhizome, 

 sometimes copiously branched. Upon the rhizome are borne 

 numerous leaves arranged in rows, which in Azolla are all alike, 

 but in Salvinia are of two kinds, floating and submerged. 



