TRANSPIRATION 181 



Other Losses. — But transpiratioai, guttatioii, bleeding, etc, 

 do not explain all the losses by the growing plant. Gases are 

 given off by the foliage and other green parts (see page 196). 

 Carbon dioxide and other compounds ai'e given off by the roots. 

 Leaves and floivsrers, twig's, fmit and branches fall and the bark 

 peels off slowly and is renewed from within (see page 52). 

 I'he total of these losses is very great. 



Most of our trees are deciduous; that is, they lose their 

 leaves in the winter ; but trees frequently lose both leaves and 

 branches during periods of drouth. Of course, the cold weather 

 of winter checks the work and growth of the plant and injure? 

 delicate structures, but the falling during periods of drontli 

 reduces the leaf surface and thereby reduces the amount of tran- 

 spiration. The falling of leaves and branches is not due to break- 

 ing off biTt due to the formation of definite layers of cell at the 

 base of the petiole or branch (see page 102). This process is 

 known as abscission. Evergreen trees shed' their leaves gradu- 

 ally throughout the entii-e life of the trees. The changing of 

 color of leaves at the approach of winter is not well understood 

 but it is evidently due to chemical changes within the cells. 



LABORATORY EXERCISES. 



Exercise 1. Soak two pieces of filter paper in 5 per cent, or stronger 

 solution of cobalt chloride. Dry thoroughly. Place an active leaf between 

 them for a few minutes and then note the color. Pink indicates absorption 

 of water. Which side of the leaf gives off most water? 



Exercise 2. Take an active growing potted plant such as Geranium or 

 Begonia. Water thoroughly, wrap in sheet rubber so as to cover both 

 pot and soil and attach about the stem of the plant. Under these conditions 

 no water can escape except through the plant. Weigh froip time to time 

 and record the loss.' 



Exercise 3. Use a Ganong potometer (Fig. 99), burette potometer 

 (Fig. 98) or other satisfactory device and determine the amount of tran- 

 spiration in a given time. 



Exercise Jf. Fill a. bottle three-fourths full of wa,ter and close with a 

 rubber stopper with two holes. Into one hole fit a leafy shoot, the lower 



