142 MANUAL OF BOTANY 



to the sclerenchyma, wMoh is never very prominent and is in 

 some cases absent. Gum passages of probably schizogenous 

 origin are found in 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. 899). 



The gametophyte is a flattened green prothallium, much 

 like that of the true ferns. It has a cushion in 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. — Heteeosporous Leptospoeangiate Fekns. 

 (Ehizocarps or HydropteridecE.) 



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 from 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 known as the seed, which is the dis- 

 tinguishing feature of Phanerogamic plants. 



The Hydropterideae or Ehizocarps 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 

 SalviniacecE and Marsileacce. Salvinia and Azolla float freely 

 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 aU alike, 

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



