CALIFORNIA PLANTS IN THEIR HOMES 



But the greatest danger that threatens these plants is 

 that the roots may not be able to supply as much water as 

 the parts above ground give off. When you shut up corn 

 leaves in a glass jar, the moisture they gave off could be 

 seen in a few minutes. In the open air evaporation is more 

 rapid. Some one has figured out the amount of water given 

 off by an acre of grass in twelve hours. It is more than 

 one hundred tons; it would cover your schoolroom floor to 

 the depth of three or four feet. Surely, our dry weather 

 plants cannot afford to part with water at this rate. 



Find out for yourselves if the plants you have collected 

 lose water as fast as the corn. Shut up in separate jars 

 equal weights of corn seedlings, and of shoots like hoar- 

 hound or tarweed with woolly leaves, twigs of live oak or 

 Eucalyptus withjhard leaves, and a fleshy plant like a cactus 

 or a Sedum. Do they give off different amounts of water ? 

 Take off the covers and leave the plants exposed to dry air 

 and the sun for a day, then weigh them again, and see which 

 have lost the most water. Next find out how much the 

 skin helps to keep in moisture. Peel off the skin from some 

 leaves, Sedum leaves are good for this; put these leaves and 

 an equal weight of entire leaves in the sun for a day or 

 two, then weigh them again. 



It is very clear now that the skin of plants control the 

 giving off of water, and that a hard or hairy skin keeps in 

 more than a delicate one, such as the corn has. Turn back 

 to the picture of the epidermis, or skin of a leaf, in Fig. 

 10. It consists, you remember, of tile-like cells fitted 

 closely together, and of pairs of cells with openings between 

 them, the pores, or stomata. Now the tile-like cells have 

 their outside walls thickened, sometimes very much, as in 

 the live oak, Eucalyptus and cactus. This thickened layer 

 contains a substance somewhat like tallow, through which 

 it is very difficult for water to pass. So most of the water 

 leaves a plant through the pores. But these little mouth- 



42 



