GROWTH PHENOMENA CONTROLLED BY INTERNAL CONDITIONS 



221 



daily increments of elongation in a leaf of Allium cepa (onion), at different stages 

 of its development, with a temperature of from 19 to 2i°C. The leaf was 

 divided into 2.5-mm. zones, and these zones are here numbered! to IX, begin- 

 ning with the basal one. The experiment began on March 8, and the increment 

 of each zone was determined after one day. The average daily increments were 

 again determined for the period from March 16 to 18, and finally for the period 

 from March 22 to 23. 



It is evident from these data that elongation soon ceased in the upper part of 

 the leaf (zone IX), and that the greatest elongation occurred in the lower 

 and yoimger part. 



Growth may sometimes result in a shortening, instead of an elongation." 

 This may arise from active growth of the parenchymatous cells of the cortex, 

 in a radial direction, in which case the vascular bundles assume an undulating 

 form. Shortening is sometimes pronounced, and it frequently has great biolog- 

 ical significance where it occurs. Many roots shorten or contract longitudinally 

 and thus draw the buds, located above, down into the soil, so that the latter are 

 protected from wounding and shielded from injurious atmospheric conditions. 

 In the case of Arum maculatum, the little tubers formed at a depth of 2 cm. are 

 subsequently drawn into the soil to a depth of 10 cm. If the tubers are 

 planted less deeply, strongly contractile roots are soon formed, which draw 

 them deeper into the soil. In the case of Crocus longiflorus (Fig. 102), only 

 slender roots are formed in the spring. Thick lateral roots with great con- 

 tracting power are formed later, and these drag the corm downward to a 

 considerable depth, and then wither away. 



' Vries, Hugo de, Ueber die Kontraktion der Wurzeln. Landw. Jahrb. 9 : 37-8o. 



1880. 



