8 
As the leaves of most plants are expanded in a drier 
medium than that which surrounds the roots, these pores 
naturally allow a large amount of moisture to escape par- 
ticularly in dry weather. If we cover a plant with a bell- 
jar we can see this moisture condensing on the sides of 
the glass, and in carefully conducted experiments we can 
actually measure the amount of water so lost. From such 
calculations we estimate that a fair-sized tree standing in 
the open may lose ten gallons of water in the course of 
a summer’s day. This loss of water from the leaves might 
at first be thought to be an accidental phenomenon due 
to the possession by plants of delicate expanded leaf struc- 
tures. As a matter of fact, however, this process of trans- 
piration is of vital importance to the plant. In the 
first place it can be shown by experiment that leaf trans- 
piration exerts a considerable amount of suction, and it is 
in part by this means that avater is raised in tall trees to 
the upper branches. In shrubs and herbaceous plants this 
suction alone would be sufficient to raise the water into the 
leaves and flowers, though the roots too are equally con- 
cerned in the ascent of sap. Nor must we consider the 
water which is given off by the leaves as so much waste. It 
is indeed important that some of the water absorbed by the 
roots should be driven off. For the water contained in the 
soil contains the mineral salts which are necessary for the 
plant, in very dilute solution, and these salts require there- 
fore to become concentrated in the plant. This can only 
be done by driving off some of the superfluous water and 
that is effected by the process of transpiration. The leaves 
are therefore acting as a condensing apparatus and thus 
performing a very needful function. . 
I trust that what I have said regarding the sensitive- 
ness of the Various parts of the plant to external forces 
and surrounding conditions will have impressed the reader 
with the power of response inherent in plants, and this 
will indicate how by artificially selecting our medium and 
method of treatment we can considerably modify the 
course of development of the plants we cultivate. 
