118 THE PLANT 
from the extent of surface considered, the number of stomata per 
square decimeter is divided by 10,000. This amounts to the number per 
square millimeter, and time may consequently be saved by using this figure 
directly. While this. formula obviously leaves much to be desired, it has 
the great advantage of making it possible to compare ecads of one species, 
or species of the same habitat or of different habitats, upon an exact basis 
of factor, function, and structure. 
ADAPTATION 
161. Modifications due to water stimuli. In adaptation, the great 
desideratum is to connect each modification quantitatively with the corres- 
ponding adjustment. This is even more difficult than to ascertain the 
quantitative relation between stimulus and functional response, a task still 
beset with serious obstacles. At the present time, little more can be done 
than to indicate the relation of marked adaptations of organs and tissues to 
the direct factors operating upon them, and to attempt to point out among 
the functions possibly concerned the one which seems to be the most prob- 
able connection between the probable stimulus and the structure under 
investigation. In the pages that follow, no more than this is attempted. 
The general changes of organs and tissties produced by water are first dis- 
cussed, and after this is given a summary of the structural features of the 
plant types based upon water-content. 
162. Modifications due to a small water supply. A water supply which 
may become deficient at any time is compensated either by changes which 
decrease transpiration, or by those that increase the amount of water 
absorbed or stored. These operate upon the form and size of the organs 
concerned, as well as upon their structure. Modifications of the form of 
leaf and stem are alike in that they lessen transpiration by a reduction of 
the amount of surface exposed to the air. Structural adaptations, on the 
other hand, bring about the protection of epidermal cells and stomata, and 
often internal cells also, from the factors which cause transpiration, or 
they anticipate periods of excessive transpiration by the storage of water 
in specialized cells or tissues. In certain extreme types the epidermis is 
itself modified for the absorption of water vapor from the air. 
163. The decrease of water loss. The following is a summary of the 
contrivances for reducing transpiration. 
1. Position of the leaf. Since the energy of a ray of sunlight is greatest 
at the sun’s highest altitudes, those leaves transpire least which are in such a 
