HOW PLANTS GROW 343 



leather. The cork-oak has a very thick outer bark which 

 is somewhat spongy but water-proof and is valuable for 

 corks for bottles. It is taken from the trees in large 

 sheets and the corks are often cut perpendicular to the 

 flat surface. 



236. Leaves. We again recall that beans, squash 

 seeds, peas, etc., have two seed leaves, and that there 

 are two very small leaves visible on the young plant in 

 the beans. These two little leaves become the first true 

 leaves of the bean after it germinates. We have noticed 

 that all the plants which we have grown formed leaves as 

 soon as they came through the soil. This being true, 

 we conclude that leaves are useful and necessary to plants. 

 To determine whether they are useful, from day to day 

 pick leaves from a few young plants, and see how it 

 affects their growth. 



Gather a dozen or more leaves of various plants and 

 compare them to see in what respects they are the same. 

 Do they all have a stem? This stem is called the petiole. 

 The broad body of the leaf is called the blade. In the 

 blade there are lines or ridges called veins. The sap flows 

 through these veins. When these veins branch and run 

 in all directions, the leaf is said to be netted-veined. 

 Oak, bean, maple, etc. have leaves that are netted- 

 veined. When the veins all run in the same direction 

 the leaf is parallel- veined. Corn and many grasses have 

 parallel-veined leaves. In general, seeds that have two 

 seed leaves produce plants with netted-veined leaves and 

 grains with one seed leaf produce plants with parallel- 

 veined leaves. 



Most leaves are covered with very fine hairs or a whitish, 

 woolly substance, usually more on the under surface than 

 on top. This extra covering helps to keep water out of 



