264 THE MICROSCOPICAL NEWS. 
Atmospheric pressure, however, will only sustain a column of 
water to a height of about 34 feet, as seen in the case of a common 
pump, whereas the height of my birch is 39 feet, and the top of a 
tree is often scores and sometimes hundreds of feet from the 
ground. 
By capillary action water ascends in wetted tubes as narrow as 
hairs (cafil/us, a hair) when they dip into water; and the height 
attained above the level of the water outside the tube, other things 
being equal, is inversely proportional to the diameter of the tubes. 
Thus, at summer temperatures, in a wetted tube one twenty-fifth 
of an inch in diameter, water will rise abgut an inch and a quarter. 
Such action is apparently insufficient to account for the rise of sap 
in trees. 
With regard to endosmose, Dutrochet found that a tied bladder 
containing a saline solution decreased in weight when placed in 
water, and that a tied bladder containing water increased in weight 
when placed in a saline solution. The saline solution passed 
through the wall of the bladder or cell in one direction quicker 
than water passed through it in the other direction. This action 
between a weaker and a stronger fluid he called endosmose. Each 
cell of a plant is the analogue of the tied bladder, and between 
one cell and another there will be the passage of fluid whenever 
the densities of the two fluids vary. This action proceeds from 
cell to cell throughout a plant, and hence may account for slow 
movements of fluids within plants. But the tips of roots do not 
necessarily dip into such an amount of water as would seem to be 
necessary were endosmose the prime cause of the ascent of sap. I 
could not discover after much careful search that the rootlets of 
my birch were in contact with actual fluid water. They seemed 
rather to terminate in minute moisture-laden air spaces. Besides, 
as I understand endosmose, its very existence depends on con- 
current exosmose. Hence, the inflow of, say, common water to a 
plant containing its minute proportions of saline matters should be 
accompanied by an outflow of a fluid of different density, either 
absolutely pure water on the one hand, or, on the other, sap con- 
taining elaborated material such as sugar. I am not aware, how- 
ever, that the outflow of either kind of fluid has ever been observed, 
or that any observer contends that it takes place. I cannot, at 
present, accept endosmose as a satisfactory and complete explana- 
tion of the outflow from the branch of my birch. 
Herbert Spencer says that wind in bending twigs, branches, and 
trunks gives alternate squeezings to one side and the other and 
corresponding extensions of tissue on the opposite sides, and that 
this action sets up currents of sap within the tissue. I have great 
respect for this eminent-and far-seeing sociologist, and I doubt not 
that wind exerts important actions upon plants, but my birch was 
