112 



OUTLINES OF PLANT LIFE. 



strands may be increased by the development of secondary wood and 

 bast. (See 1[ II4-) 



The growth of the leaves is ordinarily limited, rarely extending over 

 a single season. In a few ferns and coniferous plants the leaves live for 

 two to eight years, and some continue to grow for a longer time than 

 one season. 



Fig. 107. Fig. 108. 



Fig. 107. — A few meshes of the finest veins of a^leaf of A nthyllis. ?«, main vein : b, h, 

 branches ; a; a, a, a. closed mesh ; c, ends of the finest veins within the mesh. The 

 drawing shows only the wood bundles ; the bast bundles accompanying them and 

 the mesophyll cells filling the meshes are not shown. Moderately magnified. — Alter 

 Sachs. 



Fjg. ioS. — Ending of a vein in the mesophyll of a leaf, v, v, v, the spirally thickened 

 cells of the wood; c, t, mesophyll cells with chloroplasts; a, a, cells specialized to 

 transfer water from wood to mesophyll. Magnified 230 diam. — After Frank. 



140, Wintering, — In those plants which live from year 

 to year, producing new leaves each spring, the unfolding ot 

 these from the winter buds is due chiefly to the enlargement 

 of the rudimentary leaves 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 



