118 



LEAVES AND THEIR WORK 



full sunlight. Within a short time the leaves are found to be iu 

 positions to receive the most sunlight possible. 



Arrangement of Leaves. — A study of trees in any park, or in 

 the woods, shows that the stems of trees in thick forests are usually 

 tall and straight and that the leaves come out in clusters near the 

 top of the tree. The leaves lower down are often smaller and less 

 numerous than those near the top of the tree. Careful observa- 

 tion of any plant growing outdoors shows us that in almost every 

 case the leaves are so disposed as to get much sunlight. The ivy 



A lily, showing long, narrow 

 leaves. 



The dandelion, showing a whorled arrange- 

 ment of long, irregular leaves. 



climbing up the wall, the morning-glory, the dandelion, and the 

 burdock all show different arrangements of leaves, each presenting 

 a large surface to the light. Leaves are usually definitely arranged, 

 fitting in between one another so as to present their upper siu'face 

 to the sun. Such an arrangement is known as a leaf mosaic. Good 

 examples of such mosaics, or leaf patterns, are seen in trees having 

 leaves which come up alternately, first on one side of a branch, then 

 on the other. Here the leaves turn, by the twisting of their stalks, 

 so that all the leaves present their upper surface to the sun. In 

 the case of the dandelion, a rosette or whorled cluster of leaves 

 is found. In the horse-chestnut, where the leaves come out oppo- 

 site each other, the older leaves have longer petioles than the 



