99 



11. MICROSCOPIC CHARACTERS OF BARK. 



The published references to the microscopic structure of Eucalyptus barks seem 

 scanty in the extreme. 



General. — In " Eucalyptographia " (Mueller), under E. globuhis, we have four 

 sections, labelled as follows : — 



1. Tangential section of middle-bark (A), and inner-bark (B). 



2. Radial section of a portion of middle-bark (A), and inner-bark (B). 



3. Transverse section of a portion of inner-bark. 



4. Transverse section of a portion of middle-bark : — 



b. Bast-fibre; c. cork-cells; n. crystal-cells; m. medullary-rays; p. bast- 

 parenchyma ; s. stone-cells (214 diameters). 



Solederer even has less to say : " Stone cells, according to Moller, occur in the 

 secondary bast of species of Eucalyptus ; the stone-cells attain considerable dimensions 

 in E. corymbosa, while in other species they are only sbghtly sclerosed." (i, 355.) 



12. CALCIUM OXALATE. 



Calcium oxalate is a very common substance in plants, occurring usually in the form of needle- 

 shaped crystals (raphides) or crystal aggregates. 



Calcium oxalate crystals are undoubted excreta, representing by-products of metabolism. 

 Oxalic acid in the free state, existing as a solute in the cell sap, is believed to be poisonous, especially if 

 present in large amount, though in the various Sorrels (Oxalifs) it is abundant enough to give them their 

 characteristic taste. Even if not poisonous, free oxalic acid certainly is deleterious, since its formation 

 interferes with further cell activity, as does sugar or any other product of metabolism, unless transformed 

 into an insoluble substance or removed to other cells. Thus, the chief advantage of crystals is in 

 removing oxalic acid from solution. Sometimes it is held that crystals are beneficial in removing 

 calcium from solution, especially in calcareous soils, though this view has not met with general 

 acceptance. ... In most cases it is not. necessary or even desirable to seek a subsidiary function 

 for the excreted products of plants ; if in certain instances they have such a function, it must be regarded 

 as wholly incidental. (Coulter, Barnes and Cowles, op. cif. ii, 626.) 



Solederer, i, 352, 355, says that in the Myrtacese, oxalate of lime occurs only in 

 the form of clustered and ordinary solitary crystals — in the primary cortex and bast. 



In figures of some sections of bark ("' Eucalyptographia," under E. globuhis) 

 Mueller depicts " crystal-cells," but he does not state the composition of the crystals. 

 They are calcium oxalate. 



H. G. Smith, in a paper " On the occurrence of Calcium Oxalate in the barks 

 of the Eucalypts " (Proc. Roy. Soc. N.S.W., xxxix, 23, 1905) annojmces the presence, 

 in large quantities, of calcium oxalate in the barks of several species. It is similar in 



