CHAP, vii ATMOSPHERIC HUMIDITY 



29 



4. All absorption of nutriment from the soil, all osmosis, all transference 

 of substance, take place solely through the aid of water. The germination 

 of seeds and spores and the sprouting of sclerotia demand a supply of 

 water for their initiation. The mineral nutriment of a plant must be 

 present in a dissolved condition. 



5. The assimilation of carbon dioxide depends upon water ; it is 

 retarded in a plant that is not fully turgescent, because for one reason 

 the stomata are closed, and it ceases in a fading plant. 



6. Respiration ceases when the amount of water in a plant sinks below 

 a certain limit. 



7. The open or closed condition of the stomata, and consequently 

 transpiration, or the evaporation of water from the plant, depends upon 

 moisture. A moist condition of the leaves increases transpiration. 



8. All movements, whether due to swelling or to irritability, take place 

 only through the agency of water. 



9. The amount of water in a plant is the factor determining life or 

 death when the temperature lies outside the critical ones. Dry parts of 

 plants are the most resistant. 1 



It is therein not remarkable that death may ensue from lack of water 

 or from desiccation ; yet many plants or their parts can withstand long 

 and severe drought, 'the limits of desiccation vary greatly ; only very 

 few, mostly lowly-organized plants such as lichens, mosses, Selaginella 

 lepidophylla and allies, appear capable of withstanding almost complete 

 desiccation. 



It is likewise not surprising that no other influence impresses its mark 

 to such a degree upon the internal and external structures of the plant as 

 does the amount of water present in the air and soil (or medium), and 

 that no other influence calls forth such great and striking differences 

 in the vegetation as do differences in the supply of water. It has been 

 demonstrated by Hellriegel and others that a larger supply of water 

 yields a richer crop (of leaves, straw, fruit, roots) ; if the plant be 

 supplied with but little water, dwarf growth (nanism) ensues. 2 But it 

 may be noted that the vigour of an ordinary terrestrial plant is not pro- 

 portional to the water supplied up to an indefinite amount, for there is 

 an optimum which varies according to the nature, aeration, and other 

 characters of the soil. For the purpose of ridding itself of any excess of 

 water absorbed the plant exhibits certain devices (water-pores, gutta- 

 tion, internal bleeding) ; but there is a limit to the amount of water 

 that can be endured by a plant, for instance, plants that prefer drought 

 mostly perish soon when supplied with large quantities of water (e.g. 

 heath-plants). 



Water is conveyed to the plants by two channels the air and the 

 soil (or the water in case of aquatic plants). The power of absorbing 

 and retaining water derived from the soil will be discussed in Chapter XII ; 

 here moisture in the air and atmospheric precipitations alone will be 

 considered. 



Atmospheric humidity. 



In the atmosphere there is always some water present hi an in- 

 visible gaseous state, but the amount of this varies greatly : it rises and 

 1 See p. 23. * See Kraus, 1906 a. 



