M. Wheldale 143 



Leaves. The majority of leaves during the active vegetative period 

 are entirely without soluble pigment so far as the eye can detect. 

 Nevertheless it is possible that the leaves of anthoeyanic plants may 

 contain a small amount of pigment though it is not apparent as such. 

 The leaves of albinos, for instance, are frequently of a brighter and 

 lighter shade of green than leaves of the pigmented type even when 

 the latter are without obvious pigment, and this difference can often be 

 detected before the plant flowers. The deeper colour may, however, be 

 due to some other cause. 



When pigment is present in the veins and midrib, as is normally 

 the case in many leaves, it is usually confined to the epidermal (gene- 

 rally upper) and sub-epidermal layers. 



In leaves which are more or less permanently red {Amaranthus spp.), 

 the pigment is commonly present in the epidermis only, both upper and 

 lower, all over the leaf, but in the midrib and veins it may appear in 

 the sub-epidermal layers also. 



In red-leaved varieties {Atriplex hortensis v. rubra, Beta vulgaris 

 V. rubra, etc.) arising from a green-leaved type, the pigment is again 

 usually only present in the epidermis, both upper and under, of which 

 the cells are intensely coloured. 



It is an interesting fact that the guard-cells of the stomata in the 

 epidermis of permanently red-leaved plants and red-leaved varieties 

 are colourless when all the surrounding epidermal cells are intensely 

 coloured. 



The cases of abnormal formation of pigment in leaves may now 

 be considered. If a leaf is subjected to any kind of injury, this is 

 accompanied in many plants by a more or less intense colouration of the 

 tissues. The injury ma}' be a mechanical one, such as tearing of the 

 lamina, partial breaking of the midrib, petiole or stem, or the removal 

 of a portion of the midrib. In each case the leaf becomes pigmented 

 in the part distal to the point of injury. Sometimes the whole leaf 

 when severed from the plant and lying in a fairly moist situation will 

 turn red or purple. Injury may also be brought about by attacks of 

 insects, leaf-boring larvae, aphides and fungi. In all such cases pig- 

 mentation results. 



Sections of leaves which have been injured show a different distri- 

 bution of pigment from those which are normally coloured. Antho- 

 cyanin is most frequently present in the palisade parenchyma, often in 

 the spongy parenchyma, and more rarely in the epidermis and veins, 

 unless these were originally coloured in the normal leaf. 



