CHAP. V LIGHT 19 



2. Intense light decomposes chlorophyll. A whole series of structural 

 peculiarities have been interpreted as affording protection against too 

 intense light/ and among these are the following : 



3. The leaves of heliophytcs arc often folded (grasses, palms, screw- 

 pines), or wrinkled and bent (Myrtus bullata), while those of sciophytes 

 are flat and smooth ; this feature is well seen in many plants growing in 

 hot, dry places in the West Indies.'^ 



4. The lie of the leaves is differ oil in heliophytes and sciophytes. 

 Leaves of heliophytes are often directed sharply or even almost vertically 

 upwards (Lactuca Scariola in sunny spots, and other ' compass-plants '),^ 

 or hang vertically downwards, particularly when young (mango and 

 other tropical plants) ; whereas the leaves of sciophytes are extended 

 horizontally, as we may see in the case of dicotyledons in our beech- 

 woods. The sun's rays strike the leaves of heliophytes at acute angles, 

 and therefore lose in efficiency, but the weaker light in the forest strikes 

 the leaves of sciophytes at right angles. Young leaves are directed 

 vertically or obliquely. In dicotylous sciophytes, a leaf-mosaic ^ is 

 often formed by the juxtaposition of large and small leaves in such 

 a manner that the interstices are reduced to a minimum (Fagus, Trapa, 

 Trientalis, Mercurialis, and many other forest-herbs). In plants with 

 acicular and linear leaves, such as Juniperus and Calluna, a great difference 

 exists between heliophytes and sciophytes ; the former have erect ad- 

 pressed leaves, and the latter have spreading leaves ; the former assume 

 a permanent profile-lie, and the latter display their full surface ; these 

 orientations are necessarily assumed by the plants during the young and 

 growing stage. 



5. The photometric movements exhibited by the leaves of many plants 

 as a consequence of change in illumination may be mentioned here ; to 

 light that is intense (or rich in rays of short wave-length) leaves oppose 

 their edges, to light that is weaker (or composed mainly of rays of long 

 wave-length) they oppose their faces.^ 



6. The histology of leaves produced in the sunlight and shade respec- 

 tively is not less different. Heliophylls are often isolateral, namely, when 

 they are erect and their two surfaces are consequently equally illuminated ; 

 sciophylls are universally dorsiventral. Heliophylls have a thick pali- 

 sade tissue, which owes its thickness either to the length of the palisade 

 cells, or to the presence of additional layers of them, or to both of these 

 characters (stems with little or no foliage likewise have a thick palisade 

 tissue extending completely round them) ; sciophylls have a thinner 

 palisade tissue or none at all. Palisade cells are often directed obliquely 

 in reference to the surface ; this appears to be associated with the direc- 

 tion of the rays of incident light.' Spongy parenchyma is relatively more 

 developed in sciophylls than in heliophylls. Heliophylls are thicker than 

 sciophylls. Heliophylls have small intercellular spaces, sciophylls have 

 large ones. Heliophylls respire and assimilate more rapidly than do 

 sciophylls of the same species. 



' Wiesncr, 18766. ' Johow, 1884. * Stahl, 1S81, 1883. 



* Kerner, 1887 ; Warming, igoi. 



'- See Section III, Chapter XXX. * Heinrichcr, 1884. 



' Pick, 1 881; Johow, 1884; Heinrichcr, 1884; Habcrlandt, 1886; Warming, 

 1897. 



C 2 



