MAXIMUM EXPOSURE OF SURFACE 277 



must therefore form the starting-point of any attempt to interpret the 

 anatomical structure of the entire photosynthetic system in terms of 

 its physiological activity. It must, however, be kept in mind that 

 the radially elongated palisade-cell merely represents a particular type 

 of photosynthetic element, and that cases of tangential (longitudinal or 

 transverse) extension also occur, which have to be taken into account 

 in any anatomical exposition of the photosynthetic system that claims 

 to be comprehensive. 



It will be found convenient to begin with a consideration of the 

 green tissue of the leaf of Pinus. If a transverse section of a Pine- 

 needle is examined microscopically, the walls of the large polyhedral 

 photosynthetic cells, which are in uninterrupted contact with one 

 another laterally, are seen to possess peculiar folds or flanges projecting 

 more or less deeply into the cell-cavity. The orientation of these flanges 

 depends upon the position of the cell (Fig. 110 a); in the more deeply 

 seated cells, namely, it shows no definite relation to the leaf-surface, 

 whereas in the peripheral elements almost all the flanges are placed at 

 right angles to the surface, so that the cells in question, though poly- 

 hedral or tabular in form, come to consist of a number of branches, 

 each of which resembles a palisade-cell in shape. Commonly there are 

 two radial folds starting from opposite sides of the cell, so that in 

 transverse section the latter appears like a capital H, with an abbrevi- 

 ated cross-stroke. It might reasonably be argued that there is no real 

 analogy between this counterfeit palisade-tissue and that which consists 

 of genuine palisade-cells, but merely a superficial resemblance from which 

 no reliable conclusions can be drawn. This question cannot be settled 

 by investigation of the Pinus needle alone, since anatomically the latter 

 represents a very aberrant type of leaf. Arm-palisade-cells similar to 

 those of Pinus, however, recur in the leaves of certain Eanunculaceae 

 (e.g. Trollius europaeus, Caltlia palustris, Aconitum Napcllus, A. clis- 

 scctum, spp. of Paeonia and Anemone, etc.), where they are located 

 between upper epidermis and spongy parenchyma in precisely the 

 same manner as the typical palisade-tissue that is found in other 

 members of the same family (e.g. spp. of Ranunculus and ffelleborus, 

 Eranthis hiemalis, Acpiilegia vulgaris, etc.). There can, therefore, be no 

 doubt that these peculiar flanged photosynthetic cells actually represent 

 a remarkable modification of ordinary palisade-tissue. 



Before proceeding to examine the further consequences of this con- 

 clusion, we may pause to consider the structure of arm-palisade-tissue 

 in a little more detail. Arm-palisade-cells occur sporadically in all the 

 great divisions of vascular plants. Among Dicotyledons they are found 

 more particularly, as already stated, in the Eanunculaceae, but also in 

 the genera Sambucus (Fig. 110 b), Viburnum, Saurauja, Meliosma, 



