The Leaf-spots of Eichardia Albo-Maculata. 
137 
youngest leaves that could be found they were entirely absent. It was 
further noted that the white patches were always thinner than the rest of 
the leaf. The appearance, as seen with a hand lens, suggested that the 
epidermis had split apart in these regions ; but this explanation proved to 
be erroneous. 
Material was fixed in 90 per cent, alcohol, to show the youngest stages 
of the streaks which are visible to the naked eye, and a series extending 
from these up to nearly full-sized patches. The size of the youngest 
visible patches was about '8 or "4 mm. long by about '16 mm. wide. A 
section across such a patch is shown in Fig. 1. The reason why the leaf 
is thinner in the white region is now evident ; the palisade parenchyma 
has completely disappeared, and only the spongy mesophyll lies between 
the upper and lower epidermis. It is also seen that very few plastids 
occur in the cells of this region. It is probable that those which do 
occur are of the nature of leucoplasts, though it is not possible to dis- 
tinguish them structurally from the numerous chloroplasts of the green 
tissue. 
Fig. 2 is from a microphotograph of a somewhat later stage, in 
transverse section, and Fig. 3 represents a transverse section of a patch 
which has attained almost its full size. From the structure of these 
later stages, in which a much larger number of mesophyll cells is 
seen in the thin part of the section, the conclusion seems unavoidable 
that divisions must take place in these cells, in the plane of the leaf 
lamina, thus increasing the size of the patch. Corresponding 
divisions must also occur in the epidermal cells. At the time of 
fixing the material it was not contemplated that any cytological details 
would be required in investigating the structure, and consequently a 
cytological fixative was not used. It is therefore not surprising that no 
nuclear divisions have been seen ; nor are the nuclei of the epidermal 
cells visible, except in a few cases. For purposes of comparison pieces of 
young leaves of Richardia africana have been carefully fixed and sectioned, 
and here it has been found that the nuclei are present in all the epidermal 
cells, apparently in a perfectly healthy condition, though the cytoplasm is 
very scanty. It is likely that cytologically fixed material of B. alho- 
maculata would show them also. 
It is less easy to explain how the patch first arises than to trace its 
subsequent development. It seems probable that certain cells in the 
mesophyll become actively meristematic, a condition which does not extend 
to the palisade, but which is duplicated in both upper and lower epidermis. 
This would cause a splitting apart of the palisade in the centre of such a 
group of meristematic cells, resulting in the structure already described. 
Variegated leaves are, of course, quite a common phenomenon, though 
most frequently met with in cultivated plants. In such leaves, however, 
