424 



PHYSIOLOGY 



when the nutations are said to be induced. The latter will be particu- 

 larly discussed later (see section 4, p. 442, and section 7, p. 458). 



Rapidity. — The absolute rate of growth in the period of enlargement 

 is, of course, extremely different in different plants and under different 

 conditions. A few cases may give an idea of the upper limits. The 

 filaments of wheat stamens at the time of blooming grow for a brief time 

 at the rate of 1.8 mm. per minute, which is about the rate at which the 

 minute hand of a man's watch travels. If such a rate continued for 



24 hours, they 

 would become 

 2.5 m. long. 

 The leaf sheath 

 of the banana 

 grows at the rate 

 of 1.1 mm. and 

 that of bamboo 

 0.6 mm. per min- 

 ute. When the 

 century-plant 

 blooms (as it 

 does in 10-25 

 years), a shaft 

 about 15 mm. 

 in diameter rises 

 to a height of 

 6-8 m. at the 

 rate of about 

 15 cm. per day. 



Phase of maturation. — The phase of maturation is the final phase of 

 growth. This phase is entered upon only when enlargement has prac- 

 tically ceased; therefore its progress is not measurable, though it is quite 

 as important as the preceding. During this phase the cells attain their 

 mature form and character. In all cases the thickening of the cell wall 

 is obvious, though often slight; but sometimes it proceeds to such an 

 extreme as to be the most notable change. The thickening is never uni- 

 form, and sometimes thin and thicker spots in patterns produce an effect 

 of sculpturing that is characteristic, as in the tracheae and tracheids 

 (figs. 640, 641). Conversely the resorption of certain parts of the wall 

 may occur, as the end partitions of sieve tubes and of the components of 



Fig. 669. — Nutations of a young sunflower plant: 1 position at 

 9 A.M., 2 9:15, 3 9:30, 4 9:45. 5 10:00, 6 10:15, 7 10:30, 8 11 :oo, 

 9 11:30,70 12 M., 11 1:00 p.m., 12 2:00; from point 12 the plant 

 made a deep nod to the west till 4 P.M., then again eastward till 

 5:00, again westward till 6:00, and finally to original meridian at 

 9:00 p.m. — From data by Land. 



