124 COMPENDIUM OF GENERAL BOTANY. 



with their extensive cell-wall areas, the 4 ' arm-palisade ' ' with its 

 foldings (or incomplete cell-walls), are also formed on the principle 

 of great surface expansion. It is of course necessary that these 

 cell-wall surfaces occur on the side of the leaf exposed to sunlight. 



The arrangement of these walls and their foldings are also to be 

 considered in their relation to other requirements ; first of all they 

 serve to conduct the products of assimilation by the shortest route 

 possible, and at the same time permit light to pass to the more 

 deeply seated cell-layers. There is, no doubt, a reciprocal relation- 

 ship between the light-intensity and the perfection of the assimila- 

 tory tissue-system, in that the constant lateral position of the 

 chlorophyll-bodies in the palisade-cells (movement of the chloro- 

 phyll-grains within the palisade-cells is only an exceptional phe- 

 nomenon) is most suitable for strong light-intensities (Stahl). 

 However, the structural conformation to strong light-intensities does 

 not take a higher rank than that for conveying food-substances by 

 the shortest route possible (Haberlandt). That the latter is indeed 

 a principle of prime importance can be seen by glancing at the 

 figure of Silphium laciniatum (72) ; further, also, from the fact 

 that there is a group of plants in which the assimilating cells are at 

 the same time conducting cells ; they extend parallel to the leaf- 

 surface, either in a direction toward the leaf -base or toward the 

 median vein (leafy mosses, some monocotyledons). 



In the case of Silphium (see Fig. 72) we can see that the posi- 

 tions of the cell-wall bounding the intercellular spaces (2), although 

 eventually exposed to strong illumination, are lined with chloro- 

 phyll-grains, while the portions of the cell-wall which cross the 

 current of assimilates at right angles are free from them : this is 

 an example of the predominance of the principle of conduction. 



Finally, there are cases in which the palisade- cells are radiately 

 arranged about a vascular bundle, which unmistakably indicates 

 that the principle of conduction by the shortest route possible is of 

 prime importance. The palisade-cell placed at right angles to the 

 leaf-surface is only a very frequent special case in the series of 

 elongated assimilating cells. 



With reference to these adaptive relations we shall, with 

 HABERLANDT, place the arrangement and position of the palisade- 

 cells under the principle of conduction by the shortest path. STAHL 

 is inclined to consider the adaptation to light-intensities as the most 



