304 Why Living 



more slowly than the upper ones the 

 root bends down. 



In all these examples, since , the re- 

 sponse depends on cell growth, it is slow 

 but lasting. The part of the stem or root 

 which has once bent does not bend back. 

 You must remember that only a short 

 region at the tip of a root or stem, the 

 youngest part, can lengthen by cell en- 

 largement and cell division. When, there- 

 fore, the direction of the stimulus is 

 changed, the part which is at the mo- 

 ment the youngest part will respond. In 

 that way, by frequently changing the 

 source of the stimulus, it is possible to 

 make a stem or root grow zigzag. How 

 would you perform an experiment to 

 produce a zigzag stem? 



The discovery of auxin has had im- 

 portant results. Since the discovery of 

 the growth hormone, auxin, there has 

 been much research along these lines. 

 The Boyce Thompson Institute for Plant 

 Research at Yonkers, New York, has 

 taken a leading part in this work. A vari- 

 ety of compounds, similar to auxin in 

 chemical composition, have been made 

 in the laboratory. They, too, seem to act 

 as growth hormones, producing the most 

 startling effects on plants. Certain of 

 these cause the growth of roots on stems 

 or leaves at the point where they are 

 placed. This discovery can be made prac- 

 tical use of by people who raise plants. 

 By using "artificial auxins" it is possible, 

 too, to prevent apples from falling be- 

 fore fully ripe and to raise seedless toma- 

 toes and certain other fruits (see p. 450). 



A faster method of response. In some 

 plants the bending response depends on 

 a different mechanism which works 

 more rapidly. The sensitive plant (Mi- 



Thijigs Behave As They Do unit v 



yjwsa) is a good example. The plant has 

 compound leaves. At the point of attach- 

 ment of each leaf and leaflet in the sen- 

 sitive plant is a little swelling. The cells 

 within this swollen portion are stretched 

 to capacity with cell sap. They are said 

 to be turgid. This makes them stiff in 

 the same way that a very full hot water 

 bag is stiff. Because they are firm these 

 turgid cells help to support the leaf. Cer- 

 tain stimulations cause their cell mem- 

 branes to change in such a way that the 

 cells lose water and become soft. As a 

 result they no longer act as a support 

 and the leaf droops. The stimulus may 

 be either touch (a mechanical stimulus), 

 heat, or certain gases. It may be applied 

 at the tip of one of the leaflets, causing 

 the turgid cells in every part of the 

 plant to lose their turgidity within a 

 minute or two. It is possible that the im- 

 pulse travels along strands of proto- 

 plasm which connect the cells. After a 

 short time the cells in the swellings again 

 become turgid; the leaves recover and 

 unfold again. To study turgor try Exer- 

 cise 7. 



Unusual plant behavior. The behavior 

 of the sensitive plant is unusual. Certain 

 other plants also have unusual and inter- 

 esting behavior. Among them are some 

 of the so-called insect-eating plants 

 which catch, digest, and absorb small 

 insects. You need not go to strange for- 

 eign lands to find them. The little sun- 

 dew is often seen along the edges of 

 ponds in many parts of the United States, 

 and its relative, Venus's-flytrap, is not 

 uncommon in sandy bogs in North 

 and South Carolina. The leaf of the sun- 

 dew has sticky hairs which fold over and 

 entangle the small insect which touches 



