GUTTATION 77 
temperature and dryness of the surrounding air, sun- 
shine, etc. 
115. Many plants exude water from specially modified 
stomata (the so-called water pores) at the edges of the 
leaves when the movement of water upward has been 
strong and then, by increase of the humidity of the air, 
the evaporation has been checked rather suddenly. 
This may take place in the form of drops or even as a 
fine stream. It is called guttation. Its mechanics and 
use are not clear. 
Laboratory Exercises. Nore: Ina large class, many of these 
experiments cannot be performed by every student. In that 
ease the instructor should assign some experiments to one 
student, others to another throughout the class or should set 
up the experiments himself before the class. In either case, 
every student should make complete notes upon the experiment 
for himself. 
(a) Weigh a handful of freshly picked leaves quickly before 
they have begun to wilt. Place them in an oven at the 
temperature of about 110° C. and dry them for twelve to 
twenty-four hours. Now weigh them and note the loss in 
weight. This is almost entirely due to the evaporation of the 
water in the leaf. Calculate the percentage of water in the 
original weight. Repeat the experiment with various parts of 
the same plant such as stems, roots, flowers, fruit, seeds, etc., 
and compare the amount of water in these different parts as 
well as with the corresponding parts of other plants. 
(6) To demonstrate imbibition by cell walls, take a measured 
block of wood 5 or 6 em. long and 3 or 4 cm. square. Measure 
it when perfectly dry, i.e. after having been kept a day or two in 
an oven at 110°C. Then soak it in water (preferably warm or 
hot, to hasten the process). Now measure accurately. The 
piece will be found to have become perceptibly larger owing to 
the imbibition of water by the cell walls. Probably the first 
entrance of water into dry seeds is also due to imbibition of 
water by the cell walls and protoplasm. As soon, however, as 
the latter has imbibed enough to become liquid, osmosis 
begins to act also in the taking in of water. 
