1 78 Chapters in Modern Botany CHAP. 



observations so far, i.e. in regard to some plants, insisting, 

 for instance, on the obviousness of the phenomenon in 

 figwort, but limits them also ; these plants of Stahl's are all 

 shade-lovers, be it noted, and no doubt specially sensitive ; 

 but in not very many plants can his observations be made. 

 That palisade parenchyma is formed on both sides of leaves 

 growing -vertically is also a serious difficulty to Stahl's 

 theory. Moreover, as to chlorophyll grains, he tells us 

 they are found on the sides of the palisade cells whatever 

 the intensity or direction of the sunlight ; and that when 

 they are found on the upper surface parallel to that of the 

 leaf no change can be observed. Nor do they always 

 present their profile to the intenser light. Hence then he 

 gives up Stahl's explanation of the characteristic form of 

 the palisade cells. 



A new rationale, therefore, is wanted ; and this Haber- 

 landt finds suggested by a fresh observation, that chlorophyll 

 grains are never to be found upon the lowest wall of the 

 palisades (the bottom of the canister-shaped cells in short) . 

 Now as the current carrying off the products of assimilation 

 (out from the cell towards the bast portion of the bundle) 

 must pass through this wall, he argues that this is kept 

 free for the purpose, and that this must furnish space for 

 the chlorophyll grains, which are therefore elongated in 

 the direction of the current. In short, the form of the 

 assimilatory tissue (as also the position of the chloro- 

 phyll grains) is essentially adapted to facilitate the most 

 easy (i.e. direct and rapid) transport of the products 

 of assimilation, and not their position relative to the sur- 

 face. The frequently radial arrangement of the green 

 parenchyma in leaves or stems, like those of Cyperus of 

 Equisetum, is cited to confirm this, and so on. 



These two brief and necessarily scanty abstracts of 

 Stahl and Haberlandt respectively by no means exhaust 



