7i8 ECOLOGY 



the aerial stems of perennial herbs, and the trunks of shrubs and trees that de- 

 velop basal suckers usually are shorter lived than are the roots; while the trunks of 

 the redwood live for centuries, the roots might live indefinitely. Little is known 

 concerning the causes of varying duration. In trees the continued decay of the 

 heart-wood may be a factor, and perhaps the increasing distance year by year be- 

 tween the root tips and the upper branches may involve a decreasing water supply. 



6. THE ACCUMULATION IN STEMS OF WATER, FOOD, AND 

 WASTE PRODUCTS 



Introductory remarks. — Plants often are supposed to store food or 

 water, which they or their progeny utilize later, much as men and ani- 

 mals store food for winter use. Such a conception appears to involve 

 forethought, and should be discarded.' A better conception is that 

 unused material accumulates. If a plant manufactures more carbo- 

 hydrate or protein than it utilizes, or if it takes in more water than it 

 utilizes or gives off, the residue necessarily accumulates. Sometimes 

 such surplus food and water are subsequently utilized, but at other 

 times they remain unused. In the latter event they differ from ordi- 

 nary waste products only in that they are capable of use in constructive 

 metabolism. 



The accumulation of air and water. — Stems, as well as leaves, 

 are characterized by air spaces and air passages and sometimes by 

 capacious air chambers, especially in such hydrophytes as Myriophyl- 

 lum (fig. 791), Hippuris, and Hottonia; the oxygen and carbon dioxid 

 contained herein may be of especial advantage to submersed hydro- 

 phytes, if their gas supply is otherwise deficient. In some cases stem 

 air spaces assist in flotation (as in Hottonia). Water accumulation, 

 which has been discussed in connection with leaves, is a conspicuous phe- 

 nomenon also in stems, notably in deserts. The most remarkable 

 cases of water accumulation are in the cacti and in plants of similar 

 form (as Euphorbia, Stapelia, and Cavanillesia, the latter having barrel- 

 shaped trunks); such habits are advantageous, because of the small 

 transpiring surface in proportion to the stem volume (figs. 1035, 1040- 

 1042). During rainy periods the stems accumulate large quantities of 

 water, which become depleted during subsequent drought. The fluted 

 stem of the giant cactus (fig. 1035) undergoes accordion-like expan- 

 sions and contractions during wet and dry periods respectively, the maxi- 



■ The term, reserve food, is similarly objectionable, since it directly expresses fore- 

 thought ; a preferable expression is surplus Jood. 



