ONTOGENY 301 



namely, the fibres, the mestome elements and the intervening palisade- 

 layer all become differentiated within a primarily homogeneous procambial 

 strand. In certain species of Adiantum and in Didymochlaena sinuosa, 

 the highly characteristic arm-palisade-tissue originates from the proto- 

 derm (Fig. 110 c). In these plants the photosynthetic tissue forms the 

 superficial layer on the adaxial side of the leaf. A genuine epidermis 

 in the anatomico-physiological sense is thus absent from this face. 

 The somewhat thickened external walls of these superficial palisade-cells 

 do indeed possess the structural characteristics of outer epidermal walls ; 

 the elements in question are nevertheless primarily specialised for photo- 

 synthesis, and only perform the functions of a dermal layer in a very 

 minor degree. The same remark applies to the superficial funnel-cells 

 in the leaf of Sdaginclla. The leaf of Elodca canadensis, which consists 

 only of two cell-layers, both containing numerous chloroplasts, arises 

 except for the midrib directly from the protoderm of the stem ; the 

 midrib on the other hand is formed, as the author has shown, 150 by the 

 repeated and very regular division of a hvpodermal element of the periblem 

 (fundamental meristem) and its derivative segments. 



In conclusion, a few words may be devoted to the mode of 

 differentiation of palisade-tissue, with special reference to the stage of 

 development at which this process takes place. The differentiation of 

 palisade-cells is always initiated by the appearance of active anticlinal 

 division in approximately isodiametric mother-cells. The additional 

 walls demanded by the principle of maximum exposure of surface are 

 thus, as it were, interpolated under the eyes of the observer ; certainly 

 palisade-cells never arise, as is often alleged, by the mere elongation of 

 isodiametric meristem elements. The stage of development at which 

 these partitions appear varies in different plants. In Ficus elastica the 

 first differentiation of the palisade-tissue is contemporaneous with the 

 commencement of differentiation within the protodermal layer, and with 

 the origin of the smaller vascular strands, while at the same stage the 

 development of the mechanical system is already half completed. In 

 Garagana frutescens, on the other hand, the palisade-tissue is initiated 

 at a much earlier stage, making its appearance after the segregation of 

 the principal veins and before that of the smaller vascular bundles ; 

 the same statement applies to the green twigs of Ephedra vulgaris, in 

 which the palisade-cells and the subepidermal fibrous strands appear 

 almost simultaneously. 



