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Annie C. Maybrook. 
tissue underlying the epidermis. (6) Ventral tissue consisting of 
large colourless storage cells; in the ventral tissue of the wings two 
or three rows of these cells containing starch, and having their walls 
thickened by anastomosing bands of cellulose, the unthickened 
portions remaining as slits; in the ventral tissue of the midrib 
several layers of smaller storage cells. (7) Mycorrhiza in the 
colourless storage tissues underlying the assimilating tissue, especially 
in the tissue of the midrib. Typical thalli of Forms II and III 
shewed the following variations. 
(1) A decrease in the size of the thallus, accompanied by a 
decrease in the average number of air chambers per unit of surface 
area, and in the average size of the pore of the air chamber. These 
facts are shewn in the following table:— 
Type of Thallus. 
I. 
11 . 
III. 
Fractional light intensities ... 
•046 
•043 
•0073 
Size of typical thallus ... j p^adth 
4'5 cm. 
1 cm. 
3 cm. 
•5 cm. 
3 cm. 
•5 cm. 
Average numberof airchambers per sq. cm. 
57 
35 
32 
Size of air 
Average length of major 
axis of ellipse 
•072 mm. 
■047 mm. 
•041 mm. 
chamber pore 
Average length of minor 
axis of ellipse 
•041 mm. 
•029 mm. 
•027 mm. 
(2) A broadening of the upper narrow portion or neck of the 
assimilating filaments (Fig. 1, 3-5). 
(3) The presence of chloroplasts in the neck of the assimilating 
filaments of Form III contrasted with their limitation to the swollen 
basal portion of the filaments of Forms I and II. 
(4) The presence of chloroplasts in the epidermal cells of 
Forms II and III (ranged along the lower wall parallel to the 
surface of the thallus), contrasted with the absence of chloroplasts 
in the epidermal cells of Form I. 
The large majority of thalli of Form IV and all the thalli of 
Form V shewed marked reduction in form and structure. The 
difference in degree of moisture in the respective positions was due 
to a continuous stream of slowly dripping water in which the thalli 
of Form V were growing. 
In both forms the thalli were considerably narrower and thinner 
than in Forms I, II and III. A typical thallus of Form V however 
was narrower and more elongated than one of Form IV (Fig. 1, 
6, 7). In the majority of thalli of Form IV and all of the thafii of 
