STEMS 115 



length, but the other end of the stem, which is also 

 the oldest, constantly dies away so that the plant 

 does not greatly increase in size. The great size of 

 the bulky perennials subjects them to many dangers 

 which the underground stem does not incur, and 

 it would be difficult to set a limit to the age which 

 the latter may reach. The ravages of animals, ex- 

 tremes of heat and cold, washing away of the soil by 

 rains, or the growth of more vigorous species around 

 them, would tend to set a limit to the age which they 

 may attain. 



157. Changes in the length of life of a species. — 

 The length of life of any species is an adaptation 

 to the conditions under which it lives. Changes 

 may be brought about when the plant is introduced 

 into a new habitat. Thus the ordinary tomato is 

 an annual, as it is grown in gardens ; yet if it is 

 cultivated ia a greenhouse and sheltered from the 

 weather, it may live two or tliree times as long. 

 Bringing a species into a severer climate may have 

 the effect of reducing a perennial or a biennial to 

 an annual. 



158. Bads. — Immediately underneath the epider- 

 mis, or bark, lies the layer of living tissue which is 



