tl8 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 milHmeter, 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 tissues 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. 



I. 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 



