730 NOTES 



[e.g. Magnolia acuminata, Tropaeolum majus and Chelidoniuni majils, according 

 to Eberdt). 



3. Where palisade-cells are attached by their sides to the underlying cells, the 

 inner tangential walls may lie nearly at right angles to the leaf-surface, but are 

 nevertheless always free from chloroplasts, although any that might adhere to them 

 would be approximately hi the profile position; conversely, the lower curved 

 regions of the lateral walls, which lie almost parallel to the surface, are crowded 

 with chloroplasts, in spite of the fact that the latter are permanently exposed in the 

 surface position. Similarly, if one end of a palisade-cell projects freely into an air- 

 space, it usually bears chloroplasts over the whole of the exposed wall, although 

 the corpuscles nearest the extremity have to assume the surface position. 



4. Very frequently individual palisade-ceUs are more or less strongly curved, 

 e.g. when their lower ends are inserted upon collecting- cells or attached to a bundle- 

 sheath, or where they partly surround or over-arch a hypostomatic air-chamber; 

 sometimes the curvature is so marked, that an L-shaped element results (Scilla 

 bifolia, Fig. 107, A). As regards the distribution of the chlorophyll- corpuscles, 

 such cells do not differ from ordinary, straight palisade-cells ; all parts of the lateral 

 walls are equally crowded with chloroplasts, although a varying proportion (depend- 

 ing upon the degree of curvature) of these must assume the surface position. 



5. It has already been explained in the text (p. 291), that oblique orientation of 

 palisade-cells has usually no connection with the direction of the incident light ; 

 the frequent occurrence of oblique palisade-oells is therefore unfavourable to Stahl's 

 hypothesis. In the case of reclinate leaves [with obhque palisade-tissue ] the palisade- 

 cells slope upwards in the lower erect portion of the leaf, but downwards in the upper 

 pendulous portion ; their orientation with reference to the incident illumination 

 is, therefore, opposite in the two regions of the leaf. 



6. Stahl's interpretation does not apply to the girdle-type of photosynthetic 

 system (and related types; cf. p. 284). Here, the photosynthetic cells which are 

 situated between the vascular bundles and the leaf-surface, are elongated at right 

 angles to the surface, and must therefore be regarded as typical palisade-cells ; the 

 cells on the flanks of the bundles, on the other hand, extend parallel to the surface, 

 and every stage intermediate between these two extreme conditions is to be found 

 in a single leaf. But there can be no doubt that the orientation of all the cells of 

 a girdle depends upon the same cause ; therefore the typical palisade- elements of 

 the girdle cannot be orientated with reference either to the direction or to the inten- 

 sity of the incident illumination. The same argument apphes mutatis mutandis 

 to those cases in which the photosynthetic cells are arranged in curved series con- 

 verging towards the different bundle-sheaths (cf. Scabiosa ucrainica, fig. 121). 

 Here, again, any hypothesis as to the arrangement of the photosynthetic cells must 

 account for the orientation of every element hi each series, from the outermost, 

 which is a typical palisade- cell, to the innermost, which may be extended parallel 

 to the leaf-surface. 



7. Finally, it must not be forgotten that the palisade-cell is only a special variety 

 of the elongated photosynthetic cell ; other forms of this physiological unit may 

 be orientated hi a great variety of ways with reference to the surface of the organ 

 in which they occur. A comprehensive theory of the shape and orientation of the 

 photosynthetic cell must take all varieties of that cell into consideration; but 

 if the direction (or intensity) of the incident illumination is regarded as the deter- 

 mining factor, this condition becomes impossible of fulfilment. 



Another set of observers (Areschoug, Vesque, Kohl and Montemartini) have 

 attempted to correlate the development of palisade-tissue with the intensity of 

 transpiration, increased transpiration being supposed to favour the differentiation 



