LABORATORY PRACTICE 



2. Examine the green pulpy material thus laid bare and 



notice 



3. That it is spread out upon a woody framework composed 



of the veins. 



XIV. Examine a bit of the Calla leaf with the lens and 

 notice the numerous small dots upon its surface. These are 

 the stomata, or breathing pores, through which an interchange 

 of gases goes on between the outside air and the small 

 spaces in the green pulp of the leaf. (A single stoma may 

 be studied by examining a small piece of the epidermis 

 under the lenses of a compound microscope.) 



XV. The services rendered a plant by its ordinary, or 

 foliage leaves cannot readily be inferred without careful 

 experiment. We notice, however, that the plant sends up 

 its leaves into the air and sunshine. We notice, also, that 

 leaves developed in the dark (as e.g. on potatoes in the 

 cellar) are pale yellow, but become green again if placed in 

 the light. We conclude that the leaves need air and sun- 

 shine to do their proper work. The existence of breathing 

 pores helps us to understand that the leaves are gas-absorb- 

 ing organs ; that is, that they take something (in this case 

 carbonic acid gas) from the air, and in their turn give 

 something back (in this case oxygen). It can be demon- 

 strated that water, in the form of vapor, escapes from the 

 leaf, and that this loss must be made good by the stem 

 giving up some of its water, and the stem, in turn, receives 

 water from the roots. In this way a current of water is 

 helped to move from the roots up through the stem into the 

 leaves. 



The water thus obtained contains, dissolved in it, small 

 quantities of various substances, parts of which are combined 



