II] DIFFERENTIATION OF LEAVES 49 



reach of the spray, and never far inland. Its leathery foliage may be held 

 as halophytic. The same applies for Acrostichuni maniin, which though it 

 can be grown well without salt in greenhouses, is a constant companion of 

 the Mangroves throughout the tropics, and roots downwards into the salt 

 swamps, raising its thick leathery leaves six or eight feet into the full sun. 



The differentiation of leaves has already been mentioned in the case of 

 the " nest-habit." But it comes more prominently forward in relation to the 

 propagative function. The primitive condition has doubtless been that of 

 the " general-purposes " leaf, which is at once a nutritive organ and a base 

 for the production of sporangia. Many Ferns have retained that undifferen- 

 tiated state with all their leaves of one type to the present day. This is the 

 condition seen in the Male Shield Fern, the Bracken, the Lady Fern, and 

 the Common Polypody. But as in Flowering Plants, so here there may be 

 a differentiation of leaves for distinct purposes. The recognised types for 

 Flowering Plants are the scale-leaf, the foliage-leaf, and the sporophyll : 

 and these may all be distinguished in some Ferns. In so primitive a type 

 as Osiminda regalis certain leaves are found to have their apex arrested, 

 while the lower sheathing region is fully developed. A like condition is seen 

 in the leaves borne on the runners of the Ostrich Fern {Matteiiccia Struthio- 

 pteris). These are in fact scale-leaves, comparable with those so often seen 

 on the rhizomes of Flowering Plants, or covering their winter buds. The 

 last-named Fern shows also a strong distinction of two other types of leaf, 

 viz. the foliage-leaves which appear first in the seasonal growth, and are 

 broadly expanded and quite sterile; and the sporophylls which appear later 

 in the season, and are attenuated and fertile (Fig. 56). A like differentiation 

 is also seen in Onoclea sensibilis, and in the Hard Fern. Sometimes the 

 sterile and fertile regions are segregated as parts of the same leaf, a condition 

 seen in Thyrsopteris, Anemia, and Osiminda, while it is a very marked feature 

 in the Ophioglossaceae. Such examples, taken together with the differen- 

 tiation seen in the " Nest Ferns," illustrate a considerable degree of foliar 

 differentiation in Ferns of the same general nature as that seen in Flowering 

 Plants. It is not carried out to the same degree as in these, but still the 

 comparison is justified. 



It thus appears that in the adaptation of their sporophyte to their habitat 

 the Filicales have progressed along lines similar to those seen in Flowering 

 Plants. But the specialisation of the shoot to the habitat is not so general 

 or so exact as in them. In a very large proportion of living Ferns it appears 

 only in a low degree. There are, however, occasional examples of a relatively 

 high state of adaptation. Since such cases are isolated among various genera 

 which as a whole do not appear specialised to meet peculiar circumstances, 

 it follows that the more extreme examples have probably resulted from rela- 

 tively direct adaptation. In that case they will not form a suitable basis for 



B. 4 



