296 



PHOTOSYNTHETIC SYSTEM 



Psmd&platanus) contains a layer of very tall palisade-cells arranged in 

 groups on the top of more or less funnel-shaped collecting-cells. In a 

 " shade " leaf of the same plant the palisade-cells are shorter by one 

 half, while collecting-cells are altogether absent. Both " sun " and 

 "shade" leaves contain two layers of spongy parenchyma (Fig. 122). 

 The great majority of plants agree with the Sycamore and Beech 

 in the fact that, while bright illumination leads to an increased 

 development of palisade-tissue, the latter is never entirely lacking, 

 even when the plant or organ is growing in the deepest shade. 

 According to Stahl, however, there are instances (Lactuca Scarioln, 

 //is Pseudacorus) in which the difference between sun and shade 



leaves is more fundamental ; in 

 these cases palisade-tissue only 

 develops under the influence of 

 intense illumination, and is en- 

 tirely absent from the shade 

 leaves. No doubt, the plants in 

 question possess the normal power 

 of producing palisade-tissue ; but 

 this capacity is only awakened 

 by very powerful photic stimu- 

 lation, whereas in the majority of 

 plants relatively feeble illumina- 

 tion suffices to start the series of developmental processes which 

 culminates in the differentiation of palisade-cells. In most cases, 

 indeed, the inception of the palisade-tissue takes place quite inde- 

 pendently of illumination, while the leaf is lying in complete 

 darkness within a bud, and any effect that light may produce is purely 

 quantitative, leading to the formation of larger or more numerous 

 palisade-cells. 150 



The same circumstance also largely accounts for the fact, demon- 

 strated by Bonnier and in more detail by Wagner, that the leaves of 

 alpine plants develop a more massive palisade-tissue at high levels ; for 

 it is well known that the intensity of light increases with altitude. 151 



Stahl has pointed out that the photosynthetic system of evergreen 

 plants shows a remarkable want of plasticity. The shade leaves of 

 Vaccinium Vitis Idaca, for instance, contain at least three layers of 

 palisade-cells, while in the case of Ilex aquifolium, Vinca minor and 

 spp. of Pyrola the palisade-tissue is comparatively well-developed even 

 in the shadiest localities. The same statement applies to Bvxus semper- 

 virens, the shade leaves of which do not differ appreciably from the 

 sun leaves, either as regards their total thickness or in respect of the 

 development of the palisade-tissue. Since, moreover, many character- 



Fig. 122. 



A shade leaf (right) and a sun leaf (left) of Acer 

 Ps( udoplatanus, in T.S. 



