§1] 



KATE AND QUANTITY OF GROWTH 



351 



of the solution. Indeed, to a given solution of a particular salt, e.g. sodium 

 chloride, there corresponds a constant hygrometric state, however much the 

 temperature may vary. This hygrometric condition may be calculated by 

 the formula : 1 — na, -where saturation is taken as unity, n equals the number 

 of grammes of sodium chloride dissolved in 100 grammes of water, and a is 

 a constant factor, varying with the salt, and equal to 0.00601 in the case of 

 sodium chloride. The spores were reared in a moist chamber, whose 

 bottom was made by a plate full of the solution. 



The results of Les age's experiments are given in the fol- 

 lowing table : — 



TABLE XXXVI 



Showing Interval in Days elapsing befoee Germination when Spokes 

 OF Penicillium ake kept in Moist Chambers over Various Solutions 

 OE Sodium Chloride 



From these results it follows by calculation that the spores ger- 

 minate at the hygroscopic state of 0.82-0.84 or over, but not 

 below this limit. 



The foregoing cases, taken from the two principal groups of 

 plants, agree in showing that a certain amount of water is 

 essential to the revival of the metabolic activities which are 

 preliminary to germination. The large quantity of water 

 absorbed by the seed or spore affords the mechanism or the 

 stimulus to growth. 



Also, in the later stages of plant growth, the water both of 

 the atmosphere and of the soil is essential. The importance of 

 atmospheric moisture was shown by Rbinke ('76), who com- 

 pared the size of the leaf-stem in different hygrometric condi- 

 tions. 



The method of measuring was as follows : a young potted Datura (one 

 of the Solanacese) was placed so that the lower half of one of its stems was 

 horizontal. A fine platinum wire, suspended from a standard above, hung 

 vertically near the stem, and made one turn around it. The lower. end of 

 the wire carried a 2-gramme weight. That part of the wire which sur- 

 rounded the stem was protected by a laj'er of tinfoil. As the stem swelled 

 or grew thinner, the weight rose or fell. The vertical oscillation of the 

 weight measured the variation in ckcumference of the stem. 



