1z0 THE PLANT 
epidermis so that the amount of light and heat is diminished, and the access 
and movement of dry air impeded. While hairs assume the most various 
forms, all hairy coverings serve the same purpose, even when, as in the 
case of Mesembryanthemum, they are primarily for water-storage. Hairs 
protect stomata as well as epidermal cells: the greater number of the former 
on the lower surface readily explains the occurrence of a hairy covering on 
this surface, even though absent on the more exposed upper side. In some 
cases, hairs are-developed only where they serve to screen the stomata. 
The modifications of the stomata with respect to transpiration are numer- 
ous, yet all may be classed with reference to changes of number or level. 
With the exception of aquatic and some shade plants, the number of stomata 
is normally greater on the less exposed, i. e., lower surface. The number 
on both surfaces decreases regularly as the danger of excessive water loss 
increases, but the decrease is usually more rapid on the upper surface, which 
finally loses its stomata entirely. It has been shown by many observers that 
species growing in dry places have fewer stomata to the same area than do 
those found in moist habitats. This result has been verified experimentally 
by the writer in the case of Ranunculus sceleratus, in which, however, the 
upper surface possesses the larger number of stomata. Plants of this 
species, which normally grow on wet banks, were grown in water so 
that the leaves floated, and in soils containing approximately I0, 15, 30, 
and 40 per cent of water. The averages for the respective forms were: 
upper 20, lower 0; upper 18, lower 10; upper 18, lower II; upper II, 
lower 8; upper Io, lower 6. Reduction of number is effective, however, 
only under moderate conditions of dryness. As the latter becomes intense, 
the guard cells are sunken below the epidermis, either singly or in groups. 
In both cases, the protection is the same, the guard cells and the opening 
between them being withdrawn from the intense insolation and the dry air. 
The sun rays penetrate the chimney-shaped chambers of sunken stomata 
only for a few minutes each day, and they are practically excluded from the 
stomatal hollows which are filled with hairs. The influence of dry winds is 
very greatly diminished, as is also true, though to a less degree, for leaves 
in which the stomata are arranged in furrows. Sunken stomata often have 
valve-like projections of cuticle which reduce the opening also. Finally, in 
a few plants, water loss in times of drouth is almost completely prevented 
by closing the opening with a wax excretion. 
5. Modifications in the chlorenchym. A decrease in the size and number 
of the air passages in the leaf renders the movement of water-laden air to 
the stomata more difficult, and effects a corresponding decrease in transpi- 
ration. The increase of palisade tissue, though primarily dependent upon 
light, reduces the air-spaces, and consequently the amount of water lost. 
