THE PHYSIOLOGICAL ROLE OF WATER IN PLANTS. 17 



ftirDisLes the water necessary to establisli equilibrium in the ai'rial 

 portions of the i)huit. It is interesting" to note tliat i)hints well adapted 

 to withstand drought are nevertheless likely to profit by a supply' of 

 moisture. This is true of buckwheat. 



If the proportion of the water of the root be eonipared with that of 

 the stem it will be seen that (Ij humidity favors a general development 

 of the plant in weight, and (2) the inlluence is greatest on the aerial 

 part of the plant. For two stems of the same weight there will be the 

 greatest development of the root in a dry soil. 



The most active growth in a plant ])recedes slightly the Howeriug 

 period. A more or less abundant supply of moisture favors this growth 

 to some extent, but in any case at the time of flowering the water con- 

 tent of the plant is aj^proximatelj' the same for a given species whatever 

 the water supply. The flowering period is a time of unusual transjura- 

 tion, which produces a diminution in the proportion of internal water. 

 This is a very critical period, in which desiccation may go so far as to 

 arrest assimilation and completely check the increase in weight. If the 

 plant is furnished with sufiBcient water to carry it over this period, not 

 only will its vitality be continued during fruit bearing, but ordinarily 

 it will push out new branches and new leaves, the action of which may 

 be prolonged a considerable time after the period of flowering. The 

 beginning of flowering is, therefore, a critical period which decides the 

 weight of the final product, the fresh and dry weight doubling in a very 

 short time. Internal desiccation, however slight, is an obstacle to this 

 growth and influences the maximum product of the plant. 



Considering now the phenomena of growth as distinguished from 

 increase in weight, we see (1) that although a saturated soil produces a 

 rapid swelling of seed, germination is generally checked, principally 

 on account of a lack of aeration in the soil; (2) that a soil which is 

 about half saturated greatly favors germination ; and (3) that a dry soil 

 in which there is sufticient water to cause the seed to swell, but in which 

 that removed by evaporation is not restored, gives a germination almost 

 as rapid as a semisaturated soil, but the subsequent growth is consider- 

 ably checked through lack of water. When, therefore, the optimum con- 

 ditions are departed from growth is generally checked, but to varying 

 degrees. Plants which resist humidity well, or which are somewhat 

 indifferent, have a high optimum. Those plants which suffer from 

 humidity have two possible obstacles to growth, since their healthy 

 condition is aftected by excessive moisture as well as by excessive 

 drought. For such plants as cucurbits, castor beans, and maize the 

 o])timuin of humidity is not very high and their growth is checked to a 

 considerable degree if the optimum is exceeded. Attention is here 

 directed to a previous paper by the writer,' in which the capacity and 

 duration of growth of difi'erent organs under the influence of varying 



'Rocherches snr le role physiologiqne de I'eaii dans la vdg^tation, Auii. sci. nat. 

 Bot., ser. 7, 20 (1895) ; E. S. R., 7, p. 366. 



