262 - Multicellular Plants 



Fig. 14-3. Sleep movements in Oxa/is stricta. Left, doy position of leaflets; 

 right, night position of leaflets. Another example of turgor movement. 



Holland and the United States, the nature 

 growth substances began to become clearer. 

 The early experiments showed that replacing 

 the decapitated tip of meristem tissue re- 

 stores the growth of the cells of the stump. 

 Moreover, Went found that this effect was 

 still obtained even when a fairly thick (2 mm) 

 layer of agar was interposed between the cut 

 surfaces, which proved that some diffusible 

 substance — rather than a wave of excitation 

 — is responsible for the growth effect. 



Phytohormones, Especially the Auxins. 

 Since these early experiments, three natural 

 growth substances, or auxins, have been 

 identified; and a variety of synthetic prod- 



Fig. 14-4. Positive phototropism of the stem of a bean 

 seedling. Successive photographic exposures taken at 

 40-minute intervals. 



ucts have been found that exert analogous 

 effects. The three natural auxins — auxin a 

 (C 1S H 32 6 ), auxin b (C 1S H 3 „0 4 ), and hetero- 

 auxin (C 10 H 15 NO 2 ) — are all relatively simple, 

 readily diffusible organic compounds, al- 

 though heteroauxin has been obtained 

 mainly from animal sources. All the natural 

 auxins have been prepared in pure form, and 

 all have been synthesized artificially. Virtu- 

 ally no specificity is found in the action of 

 the different compounds; that is, all have 

 similar effects when applied to corresponding 

 parts of different plants. The auxins are pro- 

 duced primarily by meristem tissue in the 

 growing points of the plant, whence they are 

 transported to the nearby regions, where cell 

 differentiation is occurring. The auxins exert 

 various effects on the metabolism and rate of 

 multiplication of plant cells (see below). But 

 one outstanding function of the auxins is to 

 regulate — by accelerating or retarding — the 

 lengthwise growth of individual cells in the 

 growing parts of the plant. 



It is important to realize that the action of 

 the auxins upon the growth of stem and root 

 cells is exactly opposite: Lengthwise growth 

 is accelerated in the stem but retarded in 

 the root. When extra auxin is available — 

 either under experimental or natural condi- 

 tions — the stem elongates with unusual ra- 

 pidity, whereas the root lengthens at a re- 

 duced rate. Thus if a poultice containing an 

 auxin is applied to one side of a stem, faster 



