124 ENVIRONMENTAL CONDITIONS. 



The factor of duration is exceedingly important in the moisture- 

 relations of plants. While cessation of enlargement must immediately 

 ensue with incipient plasmolysis of a growing tissue, a partially plas- 

 molyzed tissue may retain vitality for a long time and may imme- 

 diately recover with the return of an adequate water-supply, provided 

 that the process of desiccation has not progressed too far. Thus, many 

 plants complete their growth in seasons and in regions where wilting 

 occurs for several hours daily, this being corrected and positive 

 growth being accomplished during the cooler or more moist hours of 

 the day. For such forms a change in the rhythm of fluctuation of the 

 water ratio might prevent maturation or reproduction, although the 

 fraction of the total period of growth represented by the total period 

 of wilting might remain unchanged. Also, the after-effect of adverse 

 conditions being, as it seems, generally more pronounced than that of 

 favorable ones, rapid fluctuations between adequate and inadequate 

 water ratios frequently result in much less growth than would have 

 occurred in a continuous period of favorable conditions, although 

 the latter were of no greater duration than the total of all the short 

 favorable periods really experienced by the plant in the first case. 

 This factor of fluctuation or variation in the environment is especially 

 difficult to consider at the present time; it is mentioned here only to 

 throw emphasis on an important phase of the duration factor which 

 will need careful investigation in the future. 



With regard to the quantitative aspect of the moisture limits which 

 plants are able to withstand, very little information is available. 

 Since the activities of the plant as a whole are the sunmiation of the 

 activities of its various parts, we must regard the primary moisture 

 condition that is effective in the control of plant activity as simply 

 the water ratio obtaining in the active tissues. But practically 

 nothing has so far been done with this dynamic aspect of the water- 

 relation. The determinations that are available bear simply upon 

 the amount of desiccation which various forms or organs may bear. 

 Many of the scattered observations on this point are presented in 

 Ewart's translation of Pfeffer's Plant Physiology under the heading 

 "Desiccation." To obtain further information bearing upon the 

 point in which we are at present interested, the whole viewpoint needs 

 to be somewhat different from that heretofore employed. The 

 moisture-contents will need to be uniformly calculated to comparable 

 terms, such as to the basis of dry weight or natural volume, and the 

 different regions of the bodies of higher plants will have to be separately 

 considered. 



In connection with the foregoing discussion of the relation between 

 the rates of entrance of water into the plant and those of its exit. 

 Woodward's^ conception of the water requirement of plants should 



'Woodward, J., Some thoughts and experiments concerning vegetation, Phil. Trans. Roy. 

 Soc. London, 21: 193-227, 1699. 



