GYMNOSPERMS 



305 



The ridging of bark is due to the fact that bark on young 

 branches and stems, when the stem enlarges and produces 

 new tissue, is so spread that longitudinal crevices are formed. 

 As more new wood and bark is built within, the spreading 

 and thickening are increased and ridges and crevices become 

 more pronounced. 



281. New vegetative growth. The production of branches 

 and needle leaves begins in the late summer and autumn. 

 When winter arrives, within the 



large buds the next year's growth 



is complete in miniature. The next 



spring the bud opens, the new 



branch extends (Fig. 248), and its 



needle leaves begin to elongate. In 



a very short time the elongation is 



complete, and within a few weeks 



the young needle leaves are full- 



grown. By observing the number 



of terminal bud scars from the tip 



to older portions of a branch the age 



of a branch may be determined. It 



will be interesting to ascertain how which has < * ld dr pped 



many years' growth can be deter- 



mined by counting the bud scars. 



282. Significance of the stem. In the series of groups of 

 plants that we have hitherto studied, no plant stem is nearly 

 so complex as that of the gymnosperm trees. In the ferns 

 we had vascular tissue, but in pines and their relatives the 

 vascular tissue is organized into a massive woody stem, one 

 often of immense height and thickness (Fig. 250). In the 

 struggle for light these plants have been highly successful. 

 Such great height and thickness as are attained involve an 

 equally well-developed root system. 



To industry the gymnosperm stem is of immense signifi- 

 cance. Pines and some other gymnosperms and many angio- 

 sperms have stems upon which much of the world's work 



FIG. 251. An old pine cone 



One half natural size 



