CHAPTER V 



LEAF-ARCHITECTURE OF FERNS 



According to the definition given in Sachs' Textbook, the Leaf is a part 

 which originates below the apex of the stem, as a lateral outgrowth. Leaves 

 arise from the primary meristem, in acropetal order, and are always exo- 

 genous. The conceptions of leaf and stem are correlative, and their tissues 

 are continuous : but the leaf usually grows more rapidly than the axis which 

 bears it, and assumes a form different from it. In all these respects the 

 leaves of living Ferns accord with the morphological definition: but they 

 commonly show continued apical growth, sometimes to an indefinite degree 

 {Gleichenia, Lygodiuui). This, together with /the preponderant size and 

 complexity of the leaf of most Ferns, has led to its designation as a 

 "Frond." But the small size of the leaf in Azolla and Salvinia shows 

 that large size is not a constant feature, nor should it in any case be held 

 to override the more general grounds of morphological definition. The 

 circinate vernation which is seen in Fern-leaves is a striking characteristic 

 of their youth, but it is not a permanent feature. It is due to hyponastic 

 growth in the young state, which is equalised as the leaf matures. A passing 

 feature such as this, though interesting as a biological device giving protec- 

 tion to the vulnerable apex, is not to be held to characterise the Fern-leaf 

 as a thing apart from other leaves : for many of these may also show 

 temporary nastic differences, though in a less marked degree. The leaves 

 of Ferns may then be ranked as parts of the same general category as the 

 leaves of other Vascular Plants. Assenting to this does not imply that all 

 such leaves were common in origin by Descent. It should always be con- 

 templated as possible that organs of foliar nature may have been initiated 

 along a plurality of phyletic lines. 



The leaves of living Ferns are markedly bifacial, a condition shown also 

 by many of the early fossils. But others belonging to the Zygopterideae 

 present the unusual condition of bearing on organs, which are ranked as 

 leaves, secondary appendages arranged in alternate pairs, so that they con- 

 stitute four longitudinal rows (Fig. j^^). At present it must suffice to mention 

 these archaic fossils, and we turn from them to the architecture of the leaves 

 of living Ferns, which conform to the dorsiventral type usual for other 

 plants. They are very variable in outline. Sometimes they appear simple 

 and unbranched : most frequently they are branched, and often in high 



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