STEMS 717 



and is ready for almost instant use upon the arrival of favorable 

 conditions. 



Trees and shrubs. — Trees and shrubs utilize a large amount of 

 energy and constructive material in the development of a protective 

 bark and of a mechanical skeleton, and they are at a disadvantage in 

 the matter of vegetative reproduction. However, growth resumption at 

 the point of growth cessation the year previous makes possible an opti- 

 mum display of foliage. Trees and shrubs are fitted for all climates 

 where there is an adequate supply of available water. In deserts tree 

 development is slight, because of constant and extreme exposure to ex- 

 cessive transpiration together with limited absorption; shrubs, however, 

 often are abundant in arid climates. In alpine and arctic regions there 

 is sufficient moisture, but its unavailability through the long winter 

 makes life conditions severe for trees, on account of continued transpi- 

 ration; shrubs are more fully developed, because they are better pro- 

 tected during the winter. There are no places too cold for trees, if 

 sufficiently protected from transpiration. Trees are absent from many 

 alpine and arctic habitats where snow lies on the ground for most of 

 the summer, but the most extensive treeless tracts are the prairies, 

 where it is probable that a combination of inadequate rainfall and ex- 

 cessive winter transpiration best accounts for the absence of trees. 



The duration of stems. — At one extreme as to duration are ephemeral 

 annuals that live but a few weeks or even days, and at the other extreme 

 are trees whose life may be measured by centuries. Annual aerial stems 

 occur not alone in annuals, but also in most biennials and in most 

 herbaceous perennials of periodic climates. Most bulbs and tubers 

 live but a year or two, the old organ dying upon the development of 

 new bulbs or tubers. Of somewhat longer life, but still relatively 

 short-lived, are various rhizomes, which advance anteriorly each year, 

 while dying posteriorly, a given portion commonly enduring for a few 

 years. Trees and shrubs remain alive much longer, appearing to have 

 a more or less definite period of life, varying with the species. While 

 in some cases a trunk may endure for a number of centuries, any given 

 part lives but a few years, namely, for the length of time elapsing before 

 the sap-wood becomes transformed into heart-wood ; in many trees 

 the dead heart-wood resists decay for centuries. 



Roots commonly equal or surpass stems in the matter of duration. The stems 

 o£ annuals and of most trees and shrubs and the underground stems of perennial 

 herbs commonly are as long-lived as are the roots, but the stems of biennials, 



