REGIONS OF GROWTH 



213 



are, however, some plants, like the Lilac shown in Figure 194, 

 in which the terminal bud is replaced by two lateral ones, 

 but such is not the rule among plants. 



Growth of Stems 



Phases of Growth. — In growth three 

 things occur: (1) the addition of new 

 cells by the meristematic tissues; (2) the 

 elongation of cells; and (3) their modifi- 

 cation into tissues. The first phase must 

 precede the other two, but elongation 

 and modification accompany each other, 

 for cells begin to modify into tissues be- 

 fore completing their elongation. In 

 short-hved plants, such as annuals, the 

 first phase is most prominent in the seed- 

 ling stage, during which most of the cells 

 upon which growth in length depends are 

 formed from the apical meristems. In 

 Corn most of the cells are formed during 

 the first three or four weeks of growth. 

 During the remainder of the growth pe- 

 riod the cells elongate and modify into 

 the tissues of the mature stem. In per- Fig. 194. — Branch of 

 ennials the three phases are repeated the Lilac, showing the ter- 

 each year as is well illustrated by the minal buds replaced by two 



, , 1 i. i T-. i • lateral ones, 



yearly growth oi trees. But even in 



trees most of the cells which have to do with the growth in 

 length are formed in the buds during the previous year; and to 

 their remarkably rapid elongation is due the conspicuous phase 

 of spring growth in which the shoot elongates and leaves and 

 flowers expand into almost full size in a few days. 



Regions of Growth. — The principal regions of growth are at 

 the apices of stems, where growth in length occurs by the addi- 

 tion of new nodes and internodes, and at the cambium layer, 

 where growth in diameter takes place. In such stems as those of 

 the Grasses, the basal portion of each internode functions for some 

 time as a meristem and thereby aids in the growth in length of the 

 internode. It is due to this feature that Corn stems, before they 



