NOTES 7i".) 



149. Cf. the papers by Stahl, G. Haberlandt, Heinricher, Volkens, Pick, Eberdt, 

 Bonnier, Leist and Wagner cited above Ll-46 J. 



150. Wben Nordhausen (Ber. 1903) objects that, in woody plants," the so-called 

 ' sun ' and ' shade ' leaves may develop independently of the direct influence of 

 light, either while still enclosed in the buds or immediately after the latter unfold." 

 he is really confirming the conclusions arrived at in the text. Nordhausen further 

 admits, that illumination has some direct influence upon the degree of development 

 of palisade- tissue. The fact that the initiation of palisade-tissue takes place in 

 the bud, and therefore in the absence of light, had been previously demonstrated 

 by the author himself. 



151. The not inconsiderable diminution in the absolute carbon-dioxide content 

 of the atmosphere with increasing altitude, and the reduced vegetative period, may, 

 as Wagner asserts, be partly responsible for the greater development of the photo- 

 synthetic system in alpine plants ; but these factors are of minor importance in 

 comparison with the intensity of illumination. 



152. While the author refers the characteristic shape and orientation of palisade- 

 cells to the physiological principles of maximum exposure of surface and expeditious 

 translocation, Stahl attempts to correlate these two features with the intensity and 

 direction of the prevailing illumination. By placing themselves on the radial walls 

 of the palisade-cells, the chloroplasts assume the profile position towards vertically 

 incident light, an arrangement which is decidedly advantageous, when the illumination 

 is direct sunshine or intense diffused light (cf. p. 272). According to Stahl, the 

 characteristic shape and orientation of palisade-cells are due to the disproportionate 

 development of the radial walls, which in turn arises out of the advantage of enabling 

 the greatest possible number of chloroplasts to assume the profile position in bright 

 vertically incident light. The principal objections to Stahl's interpretation may be 

 summarised as follows : 



1. Under natural conditions, the sun's rays never strike the surface of any par- 

 ticular leaf at right angles for any considerable period. A leaf which has assumed 

 a " heliotropieally fixed position," [cf. p. 615] receives sunlight at different angles 

 at various times of the day ; in our own latitudes a horizontally extended leaf 

 experiences no vertical insolation at all. In Central Europe (Lat. N. 45 to 55) the 

 maximum altitude of the sun (on June 21st) never exceeds 68 -5. Diffuse illumination 

 naturally meets the leaf at various angles, according to circumstances. Even those 

 rays which are, to begin with, approximately vertical to the surface, undergo so much 

 reflexion, refraction and absorption on their way through the leaf, that, even at a short 

 distance below the epidermis it would be absurd to use the terms profile- and 

 surface-positions in the sense which they bear, for example, in the case of a 

 Moss-leaf or a Fern- pro thallus. 



2. Many plants form one or more layers of palisade- tissue even when growing 

 in the deepest shade. This statement applies more particularly to evergreen plants. 

 Stahl tries to evade this difficulty by arguing that long-lived evergreen leaves have 

 to adapt themselves to several other special conditions (frost, increased mechanical 

 requirements), and are therefore unable to adjust their organisation as perfectly 

 as deciduous leaves to the prevailing conditions of illumination. But it is not obvious, 

 why an increase hi mechanical strength, and in the power of withstanding frost, 

 should interfere with adaptations which are correlated with the conditions of illu- 

 mination. Certainly, the differentiation of palisade-cells cannot increase either the 

 mechanical strength or the frost-resisting capacity of a leaf. In any case, many 

 deciduous plants also develop perfectly typical palisade-tissue in their shade-leaves 



