On the Physical Properties of Soil. 



209 



soon formed itself upon the surface, which the young plants could 

 not break through without difficulty. As gypsum is in some de- 

 gree soluble in water, this circumstance may easily contribute 

 to the formation of the crust, since some portion of the gypsum 

 becomes dissolved whenever moisture is applied, and again hardens 

 on the evaporation of the water. 



In sandy clay, no proper development took place ; a radicle and 

 plumule, about an eighth of an inch long, were indeed developed, 

 but they soon died away before breaking through the clay, a crust 

 having formed on the surface, through which the germinating seeds 

 were unable to penetrate. 



In the loamy and the stiff clay, the same phenomenon occurred, 

 but in a more distinct manner. 



In the pure clay, no development took place at all : even after 

 fourteen days had elapsed, neither plumule nor radicle was to be 

 seen, although during this period the due alternation of dryness 

 and moisture had been properly maintained ; in other respects, 

 however, the seed-corn had not suffered by it, for when after- 

 wards placed in a loose soil it grew very well. We thus see how 

 it is that the seeds of many plants are capable of lying for a 

 length of time undeveloped in the soil, and afterwards, at a later 

 period, on being placed under more favourable external circum- 

 stances, of springing up. 



In pure carbonate of lime, carbonate of magnesia, and slaty 

 marl, as well as in pure humus, garden-mould, and arable-soil, 

 the seeds germinated well; the young plants in warm weather 

 developing themselves the most beautifully in the humus and in 

 the carbonate of magnesia, in consequence, probably, of the 

 greater power of containing water which these earths possess. 



VOL, T. 



p 



