74 TENOPYR: CONSTANCY OF CELL SHAPE 
the number of its cells, and not to variations in cell size. The 
extreme limits of the number of cells of which each type of leaf 
may be made up, are obviously determined by heredity, and the 
hereditary size of the organ is due to factors of periodicity in 
growth, which determine the rate and duration of cell division. 
Cell shape 
Livingston (1901) describes the cells of Stigeoclonium as having 
two characteristic shapes: a spherical form when growing in loose 
masses on the trunks of trees, in highly concentrated media, or 
when partially desiccated; and a cylindrical form, when growing in 
filaments in extremely dilute media. Instances like the above 
might lead one to believe that cell shape is largely a matter of 
environmental conditions, such as mutual pressure, turgor as in- 
fluenced by environment, etc. However, there are unicellular 
algae, for example the desmids and diatoms, having a characteristic 
cell shape which cannot be due to pressure nor any simple turgor 
relations. 
My studies show that the cells of the lower epidermis of the 
leaves of any species of plant, in the regions between the veins, 
have a characteristic length and breadth. The cell shape may, in 
other regions, be modified to a considerable extent by various 
factors, as by the presence of stomata, veins, etc. The relative . 
length and breadth of the cells of any tissue may be the same in dif- 
ferent species or varieties, as in Linum usitatissimum and Linum 
angustifolium, or it may be somewhat different, as in Plantago 
major and Plantago lanceolata. 
Differences in the shapes of the leaves of the same plant, or of 
related species, are not correlated with corresponding differences 
in the shape of their cells. The linear leaves of Campanula 
rotundifolia and Lobelia Erinus are not composed of longer, nar- 
rower cells than those found in the round leaves, but have a 
larger number of cells in the long axis of the leaf. The cells of 
Cichorium Intybus are of the same size and shape in the lobed and 
in the constricted portions of the leaf. 
The shape of the leaf obviously cannot be the result of the dif- 
ferences in cell shape but must rather be due to factors for period- 
ically limiting the number and direction of the cell divisions in 
