352 



In the case of variegated leaves, the etiolation lies in the 

 subepidermal zone that is normally colored deep-green and may 

 extend entirely through it, though in cases where the variegation 

 is less brilliant there remains athinand sometimes unequal under- 



lying Stratum of green. The etiolated parts 

 are usually very abruptly differentiated from 

 the adjacent green, and in cross section 

 appear iike squarely inlaid pieces in mosaic 

 work. Narrow green h'nes intruded into the 

 h'ght part, or white lines intruded into the 

 green, may consist of only one or two rows 

 of cells, but these, Iike the wider bands of 

 either color, pass into the leaf at right angles 

 to its surface. Where a greenish-white stripe 

 flanks more brilliant variegation, this com- 

 monly resuHs from the pigmentation of the 



part of which is then etiolated, sometime in a step-like way, and the 

 transition of such paler and brighter strips is likely to be less angularly 

 limited than usual, while the intrusion of mesophyll green into 

 marginal variegations may be irregulär or rounded. Sometimes the 

 etiolated parts are sunken below the general leaf surface and they 

 usually shrink more rapidly, so as to become sunken, in wilting 

 leaves. 



In clear white markings there is all but complete suppression 

 of recognizable plastids: but a tinge of green which such markings 

 often show is caused by small chloroplasts, comparable with those 

 of the normal light green mesophyll, and a proper light intensity 

 causes such variegations to become nearly obliterated temporarily 

 through increase in this pigmentation. The variable and rather 

 elusive yellow pigmentation of etiolated parts seems not to result 

 from the conversion of large chloroplasts into brilliant chromo- 

 plasts, but to reside in the coarse granules of the cytoplasm, which 

 are doubtless of plastid nature. 



The normal paler ventral median band, and the general dorsal 

 light color of the leaves of many of the marginate Littaeas are 

 also due to an etiolation, apparently comparable with that of 

 variegated leaves, but less complete and usually involving fewer 

 layers of cells which are underlaid by chlorophyll-bearing tissue 

 that more gradually reaches Ihe surface at the sides of the paler 



