102 ENVIRONMENTAL CONDITIONS. 



its respective life-limits, under the given set of relations between 

 the other conditions, that a plant can exist at all.^ But the mere 

 existence of a given plant-form, its mere retention of vitality, is not 

 sufficient to give it a permanent place in the vegetation of a given 

 region; each plant must pass through various developmental stages, 

 must come to maturity, and must reproduce. Since the intensity lim- 

 its for the retention of hfe do not approach each other so closely as do 

 those for growth and reproduction, it is easy to understand that the 

 duration of the different intensities or qualities of certain factors 

 must determine whether or not a given form may come periodically 

 to maturity in any region. Though the lower temperature limit for 

 hfe in seeds, bulbs, rhizomes, and the hke, and in resistant perennials, 

 is not attained in the temperate and boreal winter, yet the ^"emperature 

 conditions for growth and the production of fruit obtain only in the 

 summer season. Similarly, the moisture conditions in a desert fall, 

 for the greater portion of the year, below the minimum for the growth 

 of many desert shrubs, these producing new leaves and flowers only 

 in and immediately following the rainy seasons. The same is true of 

 root and bulb perennials and of those annuals which succeed in the 

 desert. It is thus seen that, in regions characterized by an alterna- 

 tion of seasons of plant activity and of dormancy, the lengths of the 

 seasons of activity must determine whether or not the plant repro- 

 duces adequately; and since adequate reproduction is essential to the 

 maintenance of the form in the given region, this length of season 

 must determine whether that form succeeds or fails. 



While a mature plant, or a portion thereof, may exist in a relatively 

 inactive condition for a long period of time, in an environment whose 

 factors lie without the limits for most forms of activity (but within 

 those for the retention of life), the resisting power thus evidenced is 

 usually of but a low order when compared with that exhibited by ripe 

 seeds or spores. From a physiological point of view such bodies 

 represent merely a certain phase in the development of the plant, a 

 phase in which the life processes are even more in abeyance than during 

 the dormant periods of the mature form. An annual may play a very 

 important role in the vegetation of a region, although during the greater 



^It is of course to be borne in mind, in this connection, that an alteration in one environmental 

 condition may result in death simply b5'^ causing one or more of the vital processes to be so 

 greatly accelerated or retarded that the other given external conditions, although they have not 

 been changed, become fatal. Thus, while a given rate of water-supply may be sufficient for life 

 and growth under a low evaporating power of the air, an increase in the evaporation-rate, 

 unaccompanied by a corresponding increase in the rate of supply, may result in death. Had the 

 rate of water-supply been adequately increased as the transpiration-rate rose, such a plant might 

 have survived. It is frequently said that in such a case death is due to a condition which was 

 not altered. This simply means that internal conditions have been changed, so that an environ- 

 mental factor heretofore favorable to life becomes unfavorable, without itself changing. It is 

 in this connection that the "law of the minimum" of agriculturists has been developed. (See: 

 for example, E. J. Russell, Soil conditions and plant growth, Third ed., London, 1917, chap. II. 

 Also see: F. F. Blackman, Optima and limiting factors, Ann. Bot. 19: 281-295, 1905.) 



