80 BOTANY: PRINCIPLES AND PROBLEMS 



226. Which is darker green in color, the upper or the lower surface 

 of the leaf? Why? 



227. Why is the intensity of green color in its foliage good evidence 

 as to the health of a plant? 



228. The stems and leaves of parasitic plants, such as mistletoe and 

 dodder, are either very pale green or are some other color than green. 

 Explain. 



229. Why do autumn leaves and dying leaves often turn yellow? 



230. When cows are turned into fresh pasture in the spring, their 

 butter turns deep yellow. Why? Why does it not turn green? 



231. In order to "bleach" celery plants, gardeners cover them 

 with earth or wrap them in paper. Why is this practice effective in 

 securing the desired result? 



232. What is the orientation (vertical, horizontal, oblique or other) 

 of an ordinary leaf blade? Explain. 



233. The lower leaves on a plant or branch often hang downward 

 somewhat obliquely, in contrast to the upper leaves, which are generally 

 horizontal. Of what advantage is this fact to the plant? 



234. In many herbaceous plants the lower leaves have long petioles 

 but higher up on the stem the petioles gradually decrease in length and 

 often are quite absent in the upper leaves. Explain. 



235. Of what advantage and of what disadvantage is it to a plant 

 to have its leaves very deeply cut or lobed? 



236. Are opposite leaves usually broader or narrower than spirally 

 arranged ones? Explain. 



237. Why are plants with upright, grass-like leaves usually more suc- 

 cessful in open, sunny situations than are plants with broad, horizontal 

 leaves? 



238. Why is it easier to maintain a lawn under elms or apple trees than 

 under maples? 



239. In just what part of a tree are most of its leaves borne? Why? 



240. Why are the lower limbs of a tree growing in the forest usually 

 dead? 



241. Many plants are able to thrive in relatively deep shade on the 

 floor of the forest, where the lower limbs of the forest trees have long 

 since died. How can this be? 



