GENERAL CONCLUSIONS. 75 



tubers have a green layer, it is covered by a heavy corky layer, and but 

 little photosynthesis may result from the action of its chlorophyll. The 

 sap of these tubers carries a large proportion of dissolved material. The 

 concentration of the sap and the induration of the surface prevent anything- 

 except a very low rate of depletion, a rate which is only a fraction of that 

 of the green stems of Echinocadus of similar mass. Detached plants of 

 Ibervillea may make a growth of stems year after year, and carry a water- 

 balance that might suffice for such diminished activity for a quarter of a 

 century. One specimen has been kept under observation for eight years, 

 lost half of its water-balance during that time, and still displays seasonal 

 activity. 



It has been amply demonstrated that the shoots and seedlings of plants 

 grown in darkness do not accumulate a water-balance beyond that carried 

 by ordinary herbaceous plants. Etiolated stems contain a larger propor- 

 tion of water than others of the same species normally formed in light, 

 but the total bulk of such stems is very small. 



The floras of arid regions are so rich in specialized forms that the con- 

 clusions seem justifiable that these types bear some sort of adaptation or 

 fitness by which they have survived under the conditions presented. They 

 may be readily grouped into (l) the spinose trees, shrubs, and herbs; (2) 

 the succulents. The spinose forms are those in which the shoot shows the 

 effects of an inherited atrophy of its members and a reduction of its sur- 

 faces. The result of such reduction shows narrow leaves, short, pointed 

 branches, and short axes, these changes also being accompanied by an 

 induration of the epidermal surfaces. Many of these changes are of the 

 same character as those produced when a plant from a moist region is grown 

 under arid conditions, and the inference that these plants have come about 

 by such an inherited variation is generally allowed to pass in botanical 

 writings. The fact that many of the features of desert plants could not be 

 brought about by such direct causal action, however, suggests caution in 

 the matter, and that the origination of these forms is not capable of any 

 simple explanation or interpretation, the chief difficulty being experienced 

 in the explanation of the fact that a high evaporating action of the air of 

 a habitat acts directly upon organs and also upon the organisms as a whole 

 in a manner which may be directly antagonistic as far as morphogenic 

 action is concerned. 



The reduction of the members of the shoot and the induration of the 

 surfaces may be regarded as the more primitive or initial modification in 

 connection with desert conditions, and the enlargement or increase of tis- 

 sues accommodating a large water-balance as a secondary or consequent 

 change of a more highly specialized character. Morphological alterations 

 of the first-named kind are discernible in plants inhabiting not only arid 

 regions, but all localities in which the evaporating action of the air over- 

 balances the available absorptive capacity of the vegetation. 



