as well as the nature and action of the membranes of the living organism 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 conditions 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 mountains, 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 streamways: one comprises species of annuals and perennials with retarded stems, branches reduced to spines, small, narrow, hardened and waterproofed 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 meth-
