30 OECOLOGICAL FACTORS AND THEIR ACTION SECT, i 



falls with the temperature of the air the amount of the water that air is 

 capable of retaining in a gaseous condition varying with the temperature. 

 Cold air does not take up so much water as does hot air before becoming 

 saturated ; consequently great fluctuations occur at different times of the 

 day and year. It is not the absolute humidity of the atmosphere that is 

 of greatest moment to plant-life, but the saturation-deficit. The satura- 

 tion-deficit is the difference between the maximum and the observed 

 vapour-pressure at a given temperature, and therefore indicates the 

 additional amount of water which the atmosphere is capable of taking 

 up, or the amount that it requires to become saturated. Evaporation 

 from the surface of water at the same temperature as the air is nearly 

 proportional to the saturation-deficit. Consequently, the saturation- 

 deficit is one of the indices of the evaporating action of a climate l (if 

 temperature be also taken into consideration). The saturation-deficit is, 

 as a rule, smallest during the night and greatest during the day time. 

 But among mountains this condition is often reversed owing to the daily 

 alternation of winds blowing up and down the valleys. Transpiration is 

 largely dependent upon the saturation-deficit, yet even in a very moist 

 atmosphere the transpiration may be very considerable, because the 

 stomata remain open and the plant is heated by rays of light. 



The amount of evaporation also depends upon several other con- 

 ditions, including the temperature, the extent and other qualities of the 

 evaporating surface, so that plants very naturally have produced many 

 morphological and anatomical adaptations, enabling them to flourish 

 under various conditions of humidity. 2 The plant strives in some cases 

 to depress transpiration to a certain low limit, in other cases to promote it ; 

 certain plants, for example many sciophytes (mosses, ferns, particularly 

 Hymenophyllaceae, and others) clothing forest-soil, can assimilate only 

 in very moist air ; others are adapted to very dry air. The structural 

 features guarding against dry air and depressing transpiration are, in 

 part, identical with those guarding against intense light. 3 It must be 

 noted that it is very difficult to decide which features are to be correlated 

 with atmospheric humidity, and which with other factors co-operating at 

 the same time. The peculiarities of sciophytes mentioned on pages 19-20 

 can scarcely be attributed solely to the greater atmospheric humidity that 

 usually prevails in the shade when compared with the open, but must be 

 partially caused by weaker light, just as the peculiarities of heliophytes are 

 caused by not only intense heat and intense transpiration but also by 

 intense light. Sorauer, Mer, Vesque and Viet, Lothelier, and others have 

 found that the effects of moist air are like those of weak illumination. 

 Plants become more elongated, long-jointed, thinner, paler ; their leaves 

 are smaller and thinner, more transparent, and have their dorsiventral 

 anatomy obliterated because of the absence or feeble development of 

 palisade tissue ; the vascular bundles become weaker, the intercellular 

 spaces larger, and the mechanical tissue weaker or even suppressed. It is 

 the difference in transpiration in the one case as in the other that is respon- 

 sible for the difference in structure. 



Mosses and lichens presumably can absorb aqueous vapour from the 

 atmosphere ; but it is uncertain to what extent spermophytous plants 



1 See Hann, 1901. * See Section III. 



* See Chap. V. 



