138 



PLANT LIFE. 



of the blade, its tissues are produced. In such plants apical 

 growth ceases so early that it can 

 be observed only in the youngest 

 stages. 



170. Of branched leaves. — 

 When the leaf is to become much 

 branched, two or more new growing- 

 points develop, so that each of the 

 branches has at its apex a growing- 

 point (fig. 1 66). These growing- 

 points may arise from the apical 

 growing-point, or from the basal 

 one, or sometimes from both. The 

 branches will appear accordingly 



Fig. i6 S .-Endingof'aveinin the i n acropetal or basipetal succession, 



mesophyll of a leaf, v, v, v, the r . v t1 j • ^i 



spirally thickened ceils of the or even in both as they do in the 



xylem ; c, c, mesophyll cells with i r t«i r. .. c .«_ 



chioropiasts ; a, a, ceils of the leaves of yarrow. 1 he limits of the 

 -ifttr7rank Magnified23odiam • growing-point are even more in- 

 definite than in the stem. The cells of which the leaf is 

 composed are produced very rapidly, and at a very early 

 stage division ceases. 



171. Wintering. — In those plants which live from year to 

 year, producing new leaves each spring, the unfolding of 

 these from the winter buds is due chiefly to the enlargement 

 of cells already formed. New leaves are ordinarily produced 

 before the close of the growing season preceding that in 

 which they are expanded, and are protected in the winter 

 buds. The partly developed leaves in the bud may be flat, 

 but broad leaves are commonly folded or rolled in various 

 ways. 



172. Growth limited. — The growth of the leaves is ordi- 

 narily limited, rarely extending over a single season. In a few 

 ferns and coniferous plants the leaves continue to grow for a 

 longer time. Indeed, in the curious Welwitschia (fig. 167), 



