RELATION OF PLANTS TO WATER 27 
LESSON XI 
Relation of plants to water—transpiration 
Materials—Potted plants of sunflower, geranium, be- 
gonia, a fern, a cactus, and some succulent plants, such as 
live-forever (Sedum). Most potted plants will serve fairly 
well in this work. 
In Lesson IV it was shown that in order to make foods 
plants must obtain some material from the soil, and others 
from the air, and that in connection with this process they 
dispose of surplus water from the surfaces of the leaves. 
When the water supply is insufficient there is danger of too 
great loss of water, and plants vary greatly in their ability 
to prevent injury from this source. 
Observation and study.—By holding pieces of paper over 
representative leaves of each specimen, and by marking 
their size and calculating their area, calculate also the area 
of exposure of each plant. Tabulate the weight of one 
specimen of each kind of plant, and place it where it will 
have plenty of light and air. Weigh on successive days 
until all the plants are wilted. 
Which plants lose most weight as they wilt? Which lose 
most per square inch of exposed surface? Is wilting in pro- 
portion to the amount of water lost? What other factors 
determine wilting? Why do shade-plants wilt more readily 
than exposed plants? Water all the plants and note the 
relative rate at which they regain their normal position. 
Do those that wilted first regain their position first? 
Cut some water-plants, such as water-lily or pondweed, 
and some such land plant as branches of oak or ironweed. 
Note relative time in which each wilts. See if you can de- 
termine from the structure by means of sections why one 
should wilt before the others. Is it true that plants sub- 
jected to great exposure must reduce the amount of ex- 
