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 

 (one to 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. 

 Here each cell grows and divides for itself and in the 

 case of the first the cells soon separate, forming new 

 plants. 



(6) 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 not indispensible). Place FIG. 47. 

 this seed on moist cotton with the marked root ~^ w ^~ 

 directed downward and cover with a bell jar to experi- 

 prevent drying out. Examine at intervals of several ment (6) ' 



