126 COMPENDIUM OF GENERAL BOTANY. 



important. Nor do we deny the correlation of the adaptations. 

 The arrangements of the palisade-cells at right angles to the leaf- 

 surface is the most common position of assimilating cells, because 

 here illumination is as a rule most perfect or intense ; furthermore, 

 the adaptive development of one side favorable to light in leaves 

 illumined on one side, and the adaptive development of two sides 

 favorable to light in leaves illumined on both sides (isolateral leaves), 

 are additional evidence of this correlation, and, in general, the cor- 

 relative arrangement of typical assimilating cells. Finally, light-in- 

 tensity and anatomical structure give expression to this correlation 

 in the differences of sun-leaves and shade-leaves which develop on 

 the same plant or plant species. The difference is particularly 

 noticeable in the stronger development of the palisade-tissue in the 

 sun-leaf (Stahl). 



In addition to the two structural principles of HABEKLANDT 

 surface expansion and shortest path for the assimilates we may add 

 a third structural principle harmonizing with existing facts, namely, 

 Stahl 's principle of the adaptation to light-intensity. From the 

 above arguments we must consider this relation to light as a struc- 

 tural principle belonging to this chapter. In one respect these three 

 principles are very much alike : all are readily understood from 

 a teleological standpoint, not one is explained casual mechanically. 

 The factor light must invariably be brought into consideration. 



Below the palisade-cells of the luminous side of an ordinary 

 horizontal leaf lies the loose spongy tissue, so named, because of the 

 large intercellular spaces and irregular cell-forms. This structure, 

 which is also shown in the accompanying figures, is characteristic 

 of the lower surface of the leaf. It evidently serves to perform 

 several functions : (a) the conveying of the products of assimilation 

 to the parenchyma-sheaths of the vascular bundles ; when the proc- 

 esses of differentiation have progressed somewhat more, we may 

 also distinguish " collecting cells " (see Fig. 70, a); (5) an assimi- 

 lating activity because of the chlorophyll present. We must also 

 bear in mind the self-evident result of the bounding of numerous 

 cells by intercellular air-spaces, that is, considerable transpiration 

 must take place. The author, however, agrees with YOLKENS 

 who looks upon this transpiration as a process physically necessary 

 and which produces physiological effects, but which in itself is not 

 a physiological function. We must, however, ascribe to the 



