GROWTH PHENOMENA CONTROLLED BY INTERNAL CONDITIONS 249 



Each zone of the elongating region of the root likewise passes through a 

 grand period of growth. In an experiment in which the primary root of a seed- 

 ling of Viciafaba was marked into millimeter zones, each zone being measured 

 after one, two, three, etc., days (the temperature being from 18 to 2i.5°C), 

 the following daily increments of elongation of the youngest zone were observed. 



Fig. 102. — I, Croats longiflorus. 



Z, contracting roots. Half natural size. 



Neither does the stem 1 enlarge throughout its entire length at the same time, 

 but the elongating regions are here much longer than in the root. The stem of 

 Galium molligo has a terminal region of elongation from 2 to 4 cm. long, embrac- 

 ing from 8 to 10 internodes; this region in Aristolochia sipho is from 40 to 50 cm. 

 long and embraces from 8 to 10 internodes; in Elodea canadensis it is from 2 to 3 

 cm. long, with from 43 to 50 internodes; and in Hippuris vulgaris it is from 20 

 to 30 cm. long. The single individual zones of the stem, as is true also of the 

 root, elongate unequally, and each passes through a grand period of growth. 



Leaf enlargement 2 is mainly basipetal, the enlarging region being situated in 

 the lower portion of the organ, near the stem. In the table below are given the 



1 Askenasy, E.. Ueber eine neue Methode, urn die Vertheilung der Wachsthumsintensitat in wach- 

 senden Theilen zu bestimmen. Verhandl. Xaturhist.-Med. Ver. Heidelberg 2 : 70-153. 1880. 



: Stebler, F. G., Untersuchungen liber das Biattwachsthum. Jahrb. wiss. Bot. 11: 47-123- 1878. 



