106 MR. Н. M. WARD ON THE STRUCTURE, DEVELOPMENT, 
distinetly suberized, and it will be noticed that the cells of certain deeper layers imme- 
diately beneath the double ** palissade " series are becoming merismatic ; i. e. numerous 
division-walls appear in series parallel to the surface of the leaf, and rapidly cut off groups 
of outer cells from the mesophyll. A true meristem or cambium is formed; and, since 
(fig. 14) all the cells on the upper side become flat, tabular, or cuboid chambers filled 
with air and possessing suberized walls, the meristem must be considered a kind of phel- 
logen, producing phellem, or cork, on its exposed side*. Тһе boundaries of this phellogen 
area also correspond closely with the limits of the area covered by the thallus. Hence it 
appears necessary to infer not only that the presence of the thallus acts injuriously on 
the leaf-cells in its neighbourhood (putting aside for the moment the actually destroyed 
epidermis), but that the influence extends deeper, and the mesophyll proceeds to repair 
the damage by cutting out the injured layer with a lenticular layer of protective cork. 
If we now turn to Michelia, however, where the Alga certainly does not pierce the 
cuticle, it becomes evident that the destruction of a mass of underlying cells, and their 
subsequent exclusion by cork, takes place there also. In figs. 7, 8, & 9 are seen cross 
septa in the palisade cells, immediately beneath even small specimens of the thallus 
(fig. 8), and exactly coextensive with it; in fig. 10 this feature has been carefully 
attended to. 
Fig. 10 6 represents an accurate drawing of the cell-walls as they occur in a normal 
area of the leaf, while а is an equally accurate figure of what is seen immediately beneath 
a young thallus (left out for simplicity). 
The cuticle and epidermis, and one layer of completely suberized cells which have been 
eut off, are already sienna-brown ; and a powerfully developed phellugen has arisen beneath 
by repeated division across the palisade cells, some of which are also distorted. The 
only difference between this case and that of Citrus is that in the latter the phellogen 
arises lower down in the mesophyll. Неге, as before, the injured and reparative layers 
correspond accurately in area (fig. 8) with the surface covered by the thallus, and the 
suberized tissue outside the phellogen forms eventually a sort of slough. 
It seems clear that the injury is not due to a direct parasitic action of the thallus 
(even in the extreme case of Citrus, I do not imagine the active development to depend 
so much on absorption of food from the living leaf as on the sheltered situation enjoyed 
by the ensconced thallus), since we have seen that the latter is completely external in 
Michelia. What, then, is the nature of the influence exerted by the epiphyte ? 
It is well known that processes of cell-division are very commonly executed especially 
in the dark 7, and that in some cases the cells of lower plants do not divide to any im- 
portant extent in daylight. Now, since the shading influence of the Alga-thallus must 
be considerable when well developed, it is at least possible that the cause rests partly or 
wholly in this. 
We may probably pieture the shaded cells using up all the available material in and 
around themselves, and then, being unable to manufacture more, gradually losing their 
* Frank (‘Die Krankheiten der Pflanzen, p. 92) has also noted the healing of wounds in mesophyll by cork- 
formation. 
+ V. Sachs, Text-book, p. 674 «с. 
