PHESIDENTIAL ADDRESS — SECTION C. 61 



In attempting to interpret this contrast there are, however, 

 other features to be considered besides chlorophyll content. As 

 Willstatter and Stoll have emphasised, light penetrating a leaf 

 does not pass directly through it, but is reflected and refracted 

 in various directions by the cell walls, especially where these 

 abut on air spaces. The bright colour of yellow leaves is due 

 chiefly to light that is reflected back within the leaf and emerges 

 from it again. In a green leaf the bulk of this light is absorbed 

 before it escapes, when passing the deep green chloroplasts. 



The contrast in colour between the upper and lower sides of 

 a leaf with palisade tissue above and spongy tissue below is due 

 partly to the greater development of air-spaces and consequent 

 frequency of transverse reflecting surfaces in the spongy tissue, 

 the absence of which renders the palisade tissue more transparent 

 so that light penetrates it more deeply ; partly also to the narrow- 

 ness of the palisade cells which therefore interpose numerous 

 layers of chloroplasts in the path of rays that are travei-sing them 

 at all obliquely. It follows from the latter consideration that the 

 jiarrower the palisade cells the more complete will be the ab- 

 sorption of light passing through them, including that reflected 

 back, and therefore the darker green will the leaf appear. 



Some preliminary measurements made in my laboratory by 

 Miss E. K. Tredgold support the impression I have received from 

 my own observations that the dark green leaves characteristic of 

 our evergreen forest vegetation have narrow palisade cells, where- 

 as the lighter coloured leaves of the Proteacese, for example, have 

 wide palisade cells. 



The complementary fact seems also to be very significant, 

 namely, that wide-celled palisade tissue must also be more trans- 

 parent and allow a larger proportion of the light to penetrate it 

 unfiltered by chlorophyll. Willstatter and SloU found that 

 shading a leaf of Cucurhita by a second leaf reduced its rate of 

 assimilation to very low proportions, even when light of the in- 

 tensity of direct sunlight was employed, and they emphasise the 

 importance of leaf mosaics in this connection. What applies to 

 leaves must also apply to chloroplasts. The larger the number 

 of chloroplasts light has passed the less will be its photosynthetic 

 elficiency. It seems legitimate to infer that there is a limit to 

 the profitable depth of palisade tissue and that this limit will 

 increase with the width of the constituent cells, as well as with 

 the average intensity of light that prevails. It is hardly a mere 

 coincidence that the leaves of the Proteacegp have wide-celled 

 palisade tissue extending from both sides almost from one 

 epidermis to the other. 



Greater transpai'ency of the palisade tissue not only allows 

 light of good photosynthetic quality to penetrate to a greater 

 depth, but it also means reduced absorption bulk for bulk and 

 less heating effect. The importance of this can be gauged in view 

 of the higli internal temperatures that have been demonstrated 



