1900 | BRIEFER ARTICLES 439 
local differentation, for instance, in the asymmetrical growth of the 
wall. This power of the protoplasm might be exercised only by 
certain tissues or cells, as the “passage cells” of the endodermis, but 
probably it is by no means so restricted. 
Further, the sclution given out under pressure must be more con- 
centrated than the water that enters to take its place. This does not 
mean, though, that if, for instance filtration is from the endodermis, 
the sap in the xylem must be more concentrated than the cell-sap in 
the cortex. For all the living cells from the root hair to the wood 
May cooperate as a unit, so that if the solution in the xylem is just 
enough stronger than the water zz the ground to overcome the slight 
resistance to its passage offered by the intermediate tissues, all the 
physical requirements are satisfied. It is likewise absolutely unneces- 
sary that the deeper cortical cells have more concentrated sap than 
the root hairs, in order to withdraw water from them; although it is 
usually the case that the turgor increases from the epidermis inward. 
When an epidermal cell has taken in all the water it can hold, the 
underlying cell has only to contain cell sap osmotically stronger 
than the ground water in order that a stream may pass through into it. 
The energy which the cells exerting root pressure lose in the 
filtration of a part of their osmotically active matter is not (except, 
in part or in whole, in guttation and bleeding) a real loss to the 
plant. It is returned when the water and solute are separated by 
evaporation. So that when root-pressure raises the transpiration 
Stream, just as when any other method—capillarity, suction, imbibi- 
tion, etc.—is used, the ultimate source of energy is those rays from 
the sun that evaporate the water. In extreme cases root pressure 
amounts to an atmosphere or so. The solute necessary to explain 
this will have no discernible effect upon the amount of transpiration, 
being too insignificant beside the energy always used in evaporation, 
which is sufficient at ordinary temperatures to lift the mass i cic 
evaporated, without any change in its condition, about 140 miles.— 
4. The self-registration of photosynthesis. 
The employment of the graphic method has proved an invaluable 
aid in many fields of physiological research, and has at the same 
time been of even more general, if of less intense, utility as a feature 
of class demonstration. The very simple device described here, by 
Which the method is extended to the new field of photosynthesis, 
