STEMS AND BRANCHES AND THEIR USES 215 



yellowish or brownish spots or ridges. If one of these spots 



is examined, it is found to have been caused by a break in the 



outer surface of the bark, through which a mass of the inner 



tissue has pushed out. Such a break in the bark is called a 



lenticel. It results from the formation by a cork cambium 



layer of a loose cushion of cells whose walls are not corky. 



The pressure of this cushion bursts the outer layers of bark. 



Through the opening so made, and through the spaces between 



the cells of the cushion, the air 



penetrates to the inner cells of 



the cortex. If it were not for 



lenticels, the bark would cut off 



the living cells within, many of 



which still contain chlorophyl, 



from any supply of oxygen for 



respiration or of carbon dioxid 



for carbohydrate manufacture. 



As the bark grows thicker and its 



outer layers peel off, new lenticels 



are not usually formed. But 



some trees with thin, smooth 



bark, like the paper birch (Fig. 



132), continue to form lenticels 



even on the older parts of the 



trunk. 



234. Buds. — ■ We have already seen something of the 

 structure of buds. A bud always contains the growing end 

 of a stem or branch ; if in addition it includes only young 

 leaves and the small buds in their axils, it is a leaf bud; if it 

 includes a flower or flowers, whether or not it contains leaves 

 also, it is a flower bud. A flower bud, like a leaf bud, may be 

 terminal or axillary. Often a flower bud can be recognized 

 by its appearance ; for example, the leaf buds of the apple 

 tree are slender and pointed, the flower buds are thicker and 

 rounded. When a bud is torn apart, one is often surprised to 



Fig. 132. — A piece of birch 

 bark; the horizontal dark 

 streaks are lenticels. 



