126 



BOTANY 



A lily, showing long, narrow 

 leaves. 



are but slightly developed, and that the stem has developed far 

 more than the leaves. We have also seen (page 99) that a green 



plant will grow toward the source of 

 light, even against great odds. This effect 

 of light is seen as well in older plants. 

 Any field trip will reveal the fact that 

 in low-lying plants the leaves which are 

 shaded are often yellow and dwarfed. 

 Plants kept in poorly lighted rooms at 

 home show this. 



Arrangement of Leaves. — A careful 

 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 observation of any plant 

 growing outdoors shows us that in almost every case the leaves 

 are so disposed as to get much sunlight. The ivy climbing up the 

 wall, the morning glory, the 

 dandelion, and the burdock 

 all show different arrange- 

 ments of leaves, each pre- 

 senting a large surface to 

 the light. Leaves are usu- 

 ally definitely arranged, fit- 

 ting in between each other 

 so as to present their upper 

 surface to the sun. Such an 

 arrangement is kno^vn as a 

 leaf mosaic. Good examples 

 of such mosaics, or leaf pat- 

 terns, are seen in the alter- 

 nate-leaved trees. Here the leaves turn, by the twisting of the 

 petioles, so that all the leaves present their upper surface to the 



ihe dandelion, showing a whorled arrangement 

 of long, narrow leaves. 



