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 i.i mm. and 
that of- bamboo 
o.6mm.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 
12 
FIG. 669. Nutations of a young sunflower plant : 1 position at 
g A.M., 2 9:15, 3 9:30, 4 9:45, 5 10:00, 6 10:15, 7 10:30, a 11:00, 
9 11:30, 10 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. 
