2 yo FILICALES LEPTOSPORANGIATAE 



ferns are growing). On the under surface are borne the repr. organs 

 anlheridia ( 3 ) and arehegonia ( ) The spermatozoids swim to the 

 ova in the water which collects under the prothalli. The fert. ovum 

 developes directly into a new fern-pi. 



Two interesting modifications of the life cycle as above described 

 are known. In Pteris cretica, Dryopteris Filix-tnas, Aspidium 

 falcatuni and Todea africana, there occurs apogamy or the omission 

 of the sexual process from the life-history (see diagram in Pteri- 

 dophyta). The new fern-plant is produced from the prothallus by a 

 process of budding; a growing point developes from the cells of the 

 prothallus. The cycle thus runs : 



Fern-plant -> sporophylls -* sporangia 



t I 



*- *- prothallus 



The other case, apospory, is found in Athyrium Filix-foemina var. 

 clarissima, and in Polyslichmn augulare var. pulcherrimum, &c. 

 Here spore-formation is replaced by a process of budding which gives 

 rise to prothalli on the backs of the 1., so that the life-cycle runs 



Fern plant -* -* 



t I 



, ... , fspermatozoid - antheridium ) ., ,, 



fertilised ovum - { ^ _ arch egonium ( ^ P thall s 



The latter must not be confused with the 'vivipary' of Asplenium 

 bulhifenun, &c., where the leaf-tissue buds directly into new pi., 

 which for a time remain attached, but ultimately grow independently 

 {cf. Bryophyllum, &c.). 



A very large number of ferns are shade- and moisture-loving 

 plants. Many however are xero. and alpine forms with reduced 

 transpiration, exhibiting the familiar char, of such plants reduced 

 surface, thick cuticle, hairiness, incurving of leaves (cf. Ericaceae), 

 and even, though rarely, succulence (Polypodium adnascens, Drymo- 

 glossum carnosum, &c.). The tree ferns and many others have water 

 storage tissue in the stem. Many are epiphytic, esp. in the trop., 

 though they may be found growing in this way even in Britain. 

 Cf- Platycerium, Polypodium, &c. The spores, consisting only of 

 one cell, are much lighter than is possible for a seed, and may be 

 carried by wind to enormous distances. 



2. Hydropteridineae or Rhizocarpae. The two fams. of this 

 group, though they have much in common, are probably derived from 

 different stocks. For details cf. fams. 



As in the homosporous forms, so here the embryo gives rise 

 directly to a new leafy pi., usu. aquatic, and exhibiting a creeping 

 stem with a dorsiv. arrangement of the 1. Roots may or may not 

 be formed. The stem grows by an apical cell. The sporangia are 

 enclosed in capsular structures termed sporocarps. In the Salv. this 

 body contains one sorus only, in the Mars, more than one. The 

 sorus in the former has one kind of sporangium only, in the latter 

 usu. both. The spores germinate in water; the megaspore gives rise 

 to a small green $ prothallus which remains enclosed in the burst spore. 

 Its free surface bears a few archegonia. The microspore gives rise 



