76 



BIOLOCzY FOR BEGINNERS 



by falling leaves. These are especially plain in the horse-chestnut. 



A flower-bud always ends the growth of the stem that bore it, hence 



further growth is by lateral buds which produce a forked type of 



branching, where the flower was borne. 



Bud-scale Scars. At various places on the stem are rings of 



small scars caused by the bud-scales of previous years which were 



shed as spring activity 

 commenced, thus mark- 

 ing the first growth of 

 each year. Other mark- 

 ings are frequently met 

 with, caused by injuries 

 from weather or insects. 

 These the plant has met 

 by thickening its bark. 



INTERNAL STRUCTURE 



On cutting across the 

 stem of , any common 

 tree, the general internal 

 structure will be shown, 

 in most cases, without 

 the use of lenses. Three 

 regions can be dis- 

 tinguished easily bark, wood, and pith. A closer inspection 

 reveals a fourth, between bark and wood. This is the cambium, 

 a thin, light-colored zone of very juicy cells, which here, as in 

 the root, produces all the other tissues. 



Wood. The wood will be seen to be arranged in circles, " annual 

 rings " of alternately coarse and fine tissue, the ducts, and wood 

 fibers, while radiating from the pith and extending across these 

 rings are the pith rays that connect pith and bark. 



Bark. The bark will repay a closer scrutiny with a hand 

 lens and will be found to consist of an outer epidermal layer, 

 often variously thickened and roughened by growth; next, the 



-*- or * Srtrsy 



FIG. 15. Stem showing lenticels and diff- 

 erent kinds of buds and scars. 



