56 BOTANY: PRINCIPLES AND PROBLEMS 



which absorbs and holds rain-water and dew. In parasitic plants 

 the roots are converted into short, sucking organs, penetra- 

 ting the host-plant and withdrawing therefrom the food upon 

 which the parasite lives. 



The root and the leaf are the two most important vegetative 

 organs of the plant, and it is therefore the leaf which we shall 

 next discuss. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



124. Which is apt to be more regular and symmetrical in shape, 

 the root-system or the stem-system of a plant? Why? 



125. In a young plant growing from the seed, the root is larger and 

 better developed at first than the stem or leaves. Explain. 



126. Tap-roots are usually tapering in shape, being broadest near the 

 surface of the ground and gradually narrowing below. Explain. 



127. Do all roots grow directly downward, provided that there are 

 no obstacles in the way? Give evidence for your answer. 



128. It has been found that for most species of plants there is a rather 

 definite distance below the surf ace. of the ground at which the bulk of 

 the roots are formed. Some species are deep-rooted, some shallow, and 

 others intermediate. What physiological differences between the plants 

 can you suggest which might cause these structural differences? 



129. Why does the planting of grass on sand-dunes stop the drifting 

 of the sand? 



130. Why does a covering of grass turf or other close plant growth 

 prevent the erosion of soil on steep banks and other exposed situations? 



131. As a "soil-binder" to prevent erosion would a crop of carrots or a 

 cover of grass be better? Why? 



132. If from around a tree which has been growing in the forest all 

 its neighboring trees are cut down, it is then much more likely to blow 

 over than is a tree which has always grown in the open. Why? 



133. If a tree-trunk is partly buried, for instance through raising the 

 level of the soil about it by grading or other means, why is the tree likely 

 to die? 



134. Which type of plant do you think would withstand a drought 

 better, one with a fibrous root or one with a tap-root? Why? 



