82 PLANT BIOLOGY 



branches or in the interior of the tree top. In dense 

 foliage masses, the petioles of the lowermost or under- 

 most leaves tend to elongate to push the leaf to the light. 



On the approach of winter the leaf usually ceases to 

 work, and dies. It may drop, when it is said to be decidu- 

 ous ; or it may remain on the plant, when it is said to be 

 persistent. If persistent leaves remain green during the 

 winter, the plant is said to be evergreen. Give examples 

 in each class. Most leaves fall by breaking off at the 

 lower end of the petiole with a distinct joint or articula- 

 tion. There are many leaves, however, that wither and 

 hang on the plant until torn off by the wind ; of such 

 are the leaves of grasses, sedges, lilies, orchids, and other 

 plants of the monocotyledons. Most leaves of this char- 

 acter are parallel-veined. 



Leaves also die and fall from lack of light. Observe the 

 yellow and weak leaves in a dense tree top or in any 

 thicket. Why do the lower leaves die on house plants ? 

 Note the carpet of needles under the pines. All ever- 

 greens shed their leaves after a time. Counting back from 

 the tip of a pine or spruce shoot, determine how many 

 years the leaves persist. In some spruces a few leaves 

 may be found on branches ten or more years old. 



Arrangement of Leaves. Most leaves have a regular 

 position or arrangement on the stem. This position or 

 direction is determined largely by exposure to sunlight. In 

 temperate climates they usually hang in such a way that 

 they receive the greatest amount of lighl. One leaf shades 

 the other to the least possible degree. If the plant were 

 placed in a new position with reference to light, the leaves 

 would make an effort to turn their blades. 



When leaves are opposite the pairs usually alternate. 

 That is, if one pair stands north and south, the next pair 



