206 



HOW CROPS GROW. 



crystals of sulphate of lime. (Sitzungsberichte der Wien. 

 Akad., 37, p. 106.) Bailey ob- 

 served in certain parts of the in- 

 ner bark of the locust a series of 

 cells, each of which contained a 

 crystal. In the onion-bulb, and 

 many other plants, crystals are 

 Fig ' 25 ' abundant. (Gray's Botanical 

 Text-Book, 6th ed., Vol. II, p. 52.) 



Instances are not wanting in which there is an obvious 

 excretion of mineral matters, or at least a throwing of 

 them off to the surface. Silica, as we have seen, is often 

 found in the cuticle, but is usually imbedded in the cell- 

 wall. In certain plants, other substances accumulate in 

 considerable quantity without the cuticle. A striking ex- 

 ample is furnished by Saxifraga crustata, a low European 

 plant, which is found in lime soils. 

 The leaves of this saxifrage are en- 

 tirely coated with a scaly incrusta 

 tion of calcium and magnesium 

 carbonates. At the edges of the 

 leaf this incrustation acquires a 

 considerable thickness, as is illus- 

 trated by figure 26, a. In an anal- 

 ysis made by linger, to whom these 

 facts are due, the fresh (undried) 

 leaves yielded to a dilute acid 

 4.14% of calcium carbonate, and 

 0.82% of magnesium carbonate. 



linger learned by microscopic 

 investigation that this excretion 

 of carbonates proceeds mostly from a series or granular 

 expansions at the margin of the leaf, which are directly 

 connected with the sap-ducts of the plant. (Sitzungsbe- 

 richte der Wien. Akad., 43, p. 519.) 



Iu figure 26, a represents the appearance of a leaf, magnified 4J diam- 



') 



</' 



Fig. 26. 



