116 ENVIRONMENTAL CONDITIONS. 



can find its way into the tissues by these channels, and it follows 

 that those plants that transpire the least must exhibit the smallest 

 amount of leaf absorption. The main source of water-supply to the 

 majority of plants is of course the soil or other substratum in which 

 the plant is rooted. We may consider in greater detail the first and 

 the last of the sources just mentioned. 



In relatively large plants the diffusion of water from non-active to 

 active parts is often of great importance. Thus, as a tissue dies its 

 contained moisture may pass into other portions and there support 

 growth and other activities. A familiar example of this is exhibited 

 by bulbs, rhizomes, etc., that produce leaves, shoots, and even flowers 

 and fruits without the influx of any water from without. Water from 

 the bulb moves gradually into the more active portion and supplies 

 the moisture for growth and transpiration; here the so-called storage- 

 tissue plays the same role of water source as does the moist sub- 

 stratum in the case of ordinary rooted and absorbing plants. In many 

 instances water-bearing tissues may lose much of their water during 

 the growth period of the plant and may still retain vitality and the 

 power to absorb, so that at another season, when the external water- 

 supply is greater, such tissues may receive water in larger amounts 

 from the substratum and so return to their original turgid condition. 

 In many cacti, fleshy euphorbias, and the like, all of the water for 

 transpiration — a relatively small but nevertheless important amount — 

 and even for reproduction may be derived from the quiescent stem 

 parenchyma for long periods of time. Many water-storage plants of 

 the desert can retain vitality and maintain their reduced transpira- 

 tion for several years after they have been removed from the soil 

 and are thus able to absorb no water from without.^ 



Such isolated plants often efTect new growth and ripen fruits with 

 the return of the proper season, the conditions that bring about re- 

 newed activity in such cases being probably mainly those of tempera- 

 ture. Of course, these forms must eventually succumb to lack of 

 water, as must any other form when deprived of a water-supply, 

 but the interesting point here is simply that they may withstand the 

 absolute lack of a water-supply from without for exceedingly long 

 periods of time. 



To most plants, a root system or its analog is essejitial throughout 

 the actively transpiring phases of its development. As has just been 

 implied, such a system must likewise be present a part of the time, 

 at long intervals perhaps, even in the most extreme water-storage 

 forms. Through these water-absorbing organs the moisture of the 

 substratum finds its way to the tissues of the plant. This water often 

 traverses long distances of stem, etc., and it is thus seen that the rate at 



'MacDougal, D. T., E. R. Long, and J. G. Brown, End results of desiccation and respira- 

 tion in succulent plants, Physiol. Res., 1: 289-325, 1915. 



