GROWTH 107 
152. The amount of growth in a given length of time 
varies with the plant. Some trees in dry regions, e.g. 
Cercocarpus parvifolius, the mountain mahogany of 
Colorado, may scarcely attain a height of two meters in 
one hundred years, while a morning glory vine (Ipomoea) 
may grow 17 cm. per day, a bamboo shoot 60 cm. per 
day and a stamen of wheat 1.8 mm. per minute, i.e. at a 
rate of over 25 meters a day (but of course this rate of 
growth actually lasts only a few minutes). 
153. As growth occurs in a stem or root various 
tensions arise owing to the unequal amount of growth in 
different parts. Thus the pith of many plants (especially 
herbaceous ones) elongates considerably when removed 
from the stem and the surrounding portions shorten a 
little. While they remain in the plant the result is that 
certain parts of the plant are stretched and the pith 
compressed, which stiffens the plant just as in a turgid cell 
the stretched cell wall pressing against the osmotic 
pressure within the cell renders the cell stiff. Bark of 
trees usually shows a circumferential stretching also 
which helps to keep the stem rigid. 
Laboratory Studies. (a) Examine plants of Protococcus 
(oneto few celled) or of Spirogyra (chain of cells). Cells of 
different sizes will be found but the largest cells are 
rarely more than twice as large as the smallest _ones. iE} 
Here each cell grows and divides for itself and in the = 
case of the first the cells soon separate, forming new . 
plants. 
(b) Take a germinated seed of Indian corn, sun- 
flower or other plant and on a rapidly growing root, 
using a thread dipped in India ink, mark lines 1 mm. 
apart making the first mark 1 mm. back from the tip 
(special markers for this purpose may be bought, but 
although more convenient are notindispensible), Place fy. 47. ° 
; A . —} = 
this seed on moist cotton with the marked root PF poten 
directed downward and cover with a bell jar to experi - 
prevent drying out. Examine at intervals of several ™°** 
