SUN AND SHADE LEAVES 295 



and " shade " faces. As a rule, the " light " face coincides with the 

 adaxial side of the organ, but cases are known in which the abaxial 

 side becomes the " light " face, owing to torsion of the petiole or 

 leaf-base (Alstroemeria, Allium ursinum ; Gynerium argenteum and 

 some other grasses). In dorsiventral leaves the specialised photosyn- 

 thetic tissue is generally confined to the " light " face. In exceptional 

 cases, however (Corglus Avellana, Caccinia strigosa, Galium purpweum, 

 Nigella damascena), palisade- cells may be developed on the "shade" 

 side as well, a circumstance which proves that the palisade form is not 

 necessarily correlated with intense illumination. In these instances the 

 plant, as it were, prefers to develop a palisade-layer immediately within 

 the lower epidermis, instead of adding a deep-seated layer to the adaxial 

 palisade-tissue, the reason evidently being, that in the prevailing circum- 

 stances better illumination can be obtained close to the lower surface 

 of the leaf. 



We may next pass on to consider the influence of light upon the 

 degree of differentiation of the photosynthetic system. In the case of 

 the individual plant, and a fortiori in that of the individual photosyn- 

 thetic organ, the available light is in a high degree dependent upon the 

 environment, no less than upon the position of the organ in the shoot 

 system ; as this external illumination may itself be a very variable 

 quantity, one can readily understand why most plants display a greater 

 or less degree of plasticity in respect of the development of their 

 photosynthetic system, and, in fact, appear to be able to regulate that 

 development in accordance with the prevailing light-intensity. This 

 power of accommodation depends on the fact, that light has a direct 

 influence upon the differentiation of photosynthetic cells, in the sense 

 that an increase in the intensity of illumination not only permits of 

 increased photosynthetic activity, but also results in a more extensive 

 development of photosynthetic tissue. If, therefore, the specialised 

 photosynthetic tissue takes the form of palisade-layers as it generally 

 does among Higher Plants, then increased illumination favours both 

 the quantitative and the qualitative development of that tissue ; in 

 other words, the individual palisade-cells become larger, and additional 

 layers are also often produced. In the case of the Beech (Fagus 

 sylvatica), for example, a leaf which has developed under the influence 

 of direct sunlight contains an abaxial layer of palisade-cells, in addition 

 to the two or three adaxial layers, whereas a leaf grown entirely in the 

 shade is only provided with a solitary adaxial palisade-layer, which 

 moreover consists of relatively short cells. A " sun " leaf may be thrice 

 as thick as a " shade " leaf, solely on account of the more abundant de- 

 velopment of its palisade-tissue, the spongy tissue comprising two or 

 three layers in both cases. A " sun " leaf of the Sycamore (Acer 



