54- 7 THE POPULAR SCIENCE MONTHLY 



as well as the nature and action of the membranes of the living organ- 

 ism The opinion is hazarded that further advances in cell-mechanics 

 will await some more definite physical knowledge of the colloidal bodies 

 whose evolutions and involutions are the center of the morphological 

 interest in cytology. A systematization of the water-relations of these 

 bodies, and of the changed qualities resulting from contact and action 

 of other cell-constituents is demanded: determination of chemical 

 structure is of ultimate importance, but not so immediately necessary 

 to the physiologist, who would now welcome a return from the chemist 

 and physicist of the service rendered them earlier by botanists. 



The water-relations, now as earlier, hold the center of the stage in 

 physiology, especially in plants. In a final analysis it might be truly 

 said that it is to the immanence of this subject that the establishment 

 of the Desert Laboratory is due. It may be profitable to discuss some 

 of the problems which present themselves to those of us whose activities 

 center at that institution, and to take a glance at the living material 

 which has developed under water-conditions quite unlike those of this 

 and other regions with a moist climate. I am confident that I speak 

 with the concurrence of my colleagues when I say that whatever results 

 of importance we may have accomplished must be attributed largely to 

 the living plants available for our work and the environmental condi- 

 tions which furnish a background for our experimentation. 



If organic response to environic factors is to be taken as a potent 

 means to evolution some striking features might be expected in the 

 southwestern deserts; and when one looks up and down the slopes of 

 Tumamoc hill, or across the washes to the bajadas of the Tucson moun- 

 tains, types of vegetation not seen in regions with more moisture are 

 seen everywhere. Furthermore, it needs only a brief acquaintance with 

 the desert to know that the animals which find food and shelter in this 

 vegetation show structures and habits equally pronounced. 



Two general types of plants may be seen away from the stream- 

 ways : one comprises species of annuals and perennials with retarded 

 stems, branches reduced to spines, small, narrow, hardened and water- 

 proofed leaves, which send their roots to only a moderate depth in the 

 soil occupying a kettle-shaped mass, being of the generalized type of 

 Cannon. It may be explained at this point that the moisture of desert 

 soils available to plants is in the more superficial layers which are 

 wetted by the rains. The spinose plants now under discussion contain 

 a very small proportion of water : their bodies are hard, with a minimum 

 of development of cortex or pith, and they hold only a small amount of 

 sap in the protoplasts or suspension colloids of the cells. This juice, 

 however, is characterized by the fact that it generally contains a very 

 large proportion of salts or compounds which exert osmotic pressure. 

 The state of the cells may be determined by the use of plasmolytic metb- 



