Extraction and Measurement Techniques 29 



auxin. A piece of actively growing stem is slit longitudinally and 

 placed in a solution containing the auxin material. The auxin enters 

 along the entire length of the stem and all enlarging cells which re- 

 ceive such a stimulus respond by increased growth. Epidermal cells 

 respond to auxin by a proportionately greater growth in length than 

 in width, whereas inner cortical cells respond by growth proportionally 

 greater in width than in length (Borgstrom, 1939, p. 29). Consequently 

 the growth stimulated by an auxin causes a curvature of the stem 

 halves away from the epidermal side. Such a curvature has been 

 shown to be a function of the relative length increases of the outer to 

 inner cells (Thimann and Schneider, 1938). This differential growth 

 is apparently not entirely responsible for the curvature phenomenon 

 for, curiously enough, earlier infiltration of auxin solutions into the 

 pea stem sections before splitting produces almost no curvature at all 

 (van Overbeek and Went, 1937). 



It is obvious that the auxin materials are able to enter the pea 

 stem at any point and consequently the transport of such materials 

 within the tissues has no evident bearing on curvature. This is in con- 

 trast to the dependence upon transport in the Avena test. Some com- 

 pounds which yield no curvature in the Avena test because of poor 

 transportability will yield good curvature in the slit pea test (e.g. 

 phenylacetic acid). 



In the absence of the selective effect of an active auxin transport 

 system it is to be expected that agents other than auxins may have a 

 pronounced effect on the growth obtained. The presence of some or- 

 ganic or inorganic substances which alter growth will alter the re- 

 sults of the slit pea test. For this reason the slit pea test can be used 

 to study the effects of some compounds which modify the action of 

 auxin in growth, whereas the Avena test is not well adapted for such 

 studies. 



It should be mentioned that upon placing the slit stem in water 

 a negative curvature occurs — that is a curvature outward, toward the 

 epidermal layer. This curvature is a function of water uptake by the 

 inner cortical cells and is greatly altered by the acidity of the solution 

 (van Overbeek and Went, 1937). Minute amounts of acid cause very 

 large negative curvatures. For this reason the slit pea test is not well 

 adapted for studies in which pH differences will exist between dishes. 



The pea plant produces small scale-leaves at the first two nodes 

 and a trifoliate leaf at the third node. The actively growing section of 

 stem between the second scale node and the leaf node is the material 

 used in the slit pea test. The plants are grown in the darkroom in 

 order to retain maximum sensitivity to auxins. However, plants grown 



