GROWTH SUBSTANCES FOR NORMAL GROWTH 



107 



up with the fundamental process of growth. There are at least 

 three different hypotheses concerning the nature of the first step 

 in growth: 



1. ELASTICITY. — Accordiiig to the first hypothesis, the growing 

 cell wall must be extended elastically by turgor pressure first; it 

 would at the same time, of course, become thinner. The original 

 thickness is regained either by the incorporation of new particles 



h£ 



2> b 



/f 



]/ 



Fig. 35. — Method of determining elastic and plastic extensibility in Avena 

 coleoptiles. The coleoptiles were cut away from the seed, the young foliage 

 leaves pulled out, and the resulting hollow cylinders fastened on pins above 

 metal plates. The tips were removed from series a one hour before the experi- 

 ment started, thus depriving them of a growth-hormone supply. The tips were 

 left intact in series b until the moment the experiment was begun, hence growth 

 hormone was present (indicated by stippling). Then the tips were removed 

 from series b and 250 mg. weights were placed on the ends of coleoptiles in both 

 series. After one hour the curvature was about the same in both series (ai and 

 bi). When the weights were removed, the curvatures decreased. Series a 

 without growth hormone retained only 9.3° curvature (a;). Series b with 

 growth hormone retained an angle of 17.3° (b-j). Hence, plastic extensibihty 

 was greater in coleoptiles containing growth hormone. (Adapted after Heyn, 

 19316.) 



iintussusce'ption) or by the laying down of new layers {apposition) . 

 The force that stretches the cell is, according to this hypothesis, 

 produced by turgor, and the first step in growth is reversible and 

 is not concerned with an increase in substance of the cell wall. A 

 change in the rate of growth, whether general or unilateral, can 

 come about by modification of the extending forces (turgor 

 pressure) or of the elastic extensibility. 



2. PLASTICITY. — In the second case, turgor pressure also may 

 be considered the force behind the growth of the cell wall. While 

 the extension considered in the first hypothesis is elastic, i.e., 

 reversible, in this case it is plastic, i.e., not reversible. The first 

 step in growth is not concerned with an increase of cell-wall 



