CHAPTER VIII 



HOW PLANTS ARE INFLUENCED BY THEIR 

 SURROUNDINGS 



WE have already learned that the needs of the plant 

 correspond closely to our own physical needs: they 

 consist, namely, of food, water, light, air and warmth. 

 Depriving a plant of any one of these kills it : on the 

 other hand, an excess of any of these is usually as bad 

 as an insufficiency. In situations where all these needs 

 are properly supplied, plants grow to perfection; but 

 such situations are limited in extent, and, in the fierce 

 competition to which plants are subject, many are 

 forced to grow in unfavorable situations or not at all. 

 How they make the best of these situations and adapt 

 themselves to them is an interesting study, which is 

 best approached by experimental methods 1 . 



WATER. Let us begin with the influence of water on 

 the plant. An excellent plant to illustrate this is the 

 Potato. A potato contains sufficient water to sprout 

 by itself , even when kept rather dry: if a potato which 

 has just started to sprout is placed where it is not 

 exposed to direct sunlight, it will continue to grow 



1 See an article by Webber in the Year Book of the U. S. Dept. of Agricul- 

 ture for 1895. 



(326) 



