TENOPYR: CONSTANCY OF CELL SHAPE 57 
since it varies according to the height of the stem. Moreover, in 
each leaf, the cells at the apex, at the base, and midway between 
these two regions differ in size. 
Sierp used the Schwendener-Ambronn method for finding the 
average cell size. This method gives the average area of the cell 
from a surface view, or of its cut surface in longitudinal or cross 
section. A camera-lucida drawing is made of a group of the cells 
of the epidermis, or of the cells of any other tissue, of each of the 
organs compared. These drawings are made on a piece of Bristol 
board of as uniform thickness as possible, the area and weight of 
this piece of Bristol board being carefully noted. The drawing 
of the group of cells is then cut out and weighed also. The product 
of the weight, times the area of the original piece of Bristol board, 
divided by the weight of the camera-lucida drawing, gives the area 
of the group of cells drawn. This area, divided by the number of 
cells in the group, gives the average area of the cell. The area of 
the organs compared is ascertained in a similar way. 
Possibility of error lies in the uneven thickness of the Bristol 
board, in its absorption of moisture from the atmosphere, and in 
the lack of precision in cutting inside or outside of, not through, 
the outline of the group of cells. However, as many tissue cells, 
especially those of the epidermis, are very irregular, this method of 
determining cell size would seem preferable to that of basing one’s 
comparisons on the average length or average width of the cell or 
on the product of ‘these two dimensions, as if the cells were indeed 
rectangular. 
The work of Sanio, Amelung, Jakushkine and Wawilow, and 
Conklin seems to warrant the conclusion that while the cells 
of an organ may differ greatly in size for various and obscure 
reasons, the average cell size of an organ is constant for the species 
or variety. The work of Gates, Keeble, Neilson Jones and Sierp 
shows that hereditary differences in body size of varieties or species 
may be due to corresponding or reverse differences in cell size, in 
cell number, or to both these factors. 
THE PROBLEM OF CELL FORM 
The further questions suggest themselves: Is there any correla- 
tion between the shape of the plant organ and the shape of its con- 
